[#8168] Fix NPE in EntityCombinedFileset by initializing hiddenProperties field - #8238

…Properties field (apache#8238)

### What changes were proposed in this pull request?
In `EntityCombinedFileset.java`, the `properties()` method directly
called `hiddenProperties.contains()` without null checking, which could
throw a `NullPointerException` if `hiddenProperties` was not
initialized. This PR updates the code to initialize `hiddenProperties`
with an empty set and adds null-safe handling in the
`withHiddenProperties()` method.

### Why are the changes needed?
Without this fix, calling `properties()` on an `EntityCombinedFileset`
instance before setting hidden properties could cause NPEs, breaking
fileset property retrieval. Adding proper initialization ensures that
the object is always in a valid state and handles missing hidden
properties gracefully.

Fixes apache#8168

### Does this PR introduce any user-facing change?
No. Valid requests behave the same. Cases where `hiddenProperties` was
not explicitly set are now safely handled without throwing exceptions.

### How was this patch tested?
* Added unit tests to verify that calling `properties()` without setting
hidden properties does not cause exceptions
* Added tests to ensure null input to `withHiddenProperties()` is
handled safely
* Added tests to verify that hidden property filtering works correctly
when properties are set
* Verified that existing functionality remains unchanged

…Properties field (apache#8238)

### What changes were proposed in this pull request?
In `EntityCombinedFileset.java`, the `properties()` method directly
called `hiddenProperties.contains()` without null checking, which could
throw a `NullPointerException` if `hiddenProperties` was not
initialized. This PR updates the code to initialize `hiddenProperties`
with an empty set and adds null-safe handling in the
`withHiddenProperties()` method.

### Why are the changes needed?
Without this fix, calling `properties()` on an `EntityCombinedFileset`
instance before setting hidden properties could cause NPEs, breaking
fileset property retrieval. Adding proper initialization ensures that
the object is always in a valid state and handles missing hidden
properties gracefully.

Fixes apache#8168

### Does this PR introduce any user-facing change?
No. Valid requests behave the same. Cases where `hiddenProperties` was
not explicitly set are now safely handled without throwing exceptions.

### How was this patch tested?
* Added unit tests to verify that calling `properties()` without setting
hidden properties does not cause exceptions
* Added tests to ensure null input to `withHiddenProperties()` is
handled safely
* Added tests to verify that hidden property filtering works correctly
when properties are set
* Verified that existing functionality remains unchanged

…Properties field (apache#8238)

### What changes were proposed in this pull request?
In `EntityCombinedFileset.java`, the `properties()` method directly
called `hiddenProperties.contains()` without null checking, which could
throw a `NullPointerException` if `hiddenProperties` was not
initialized. This PR updates the code to initialize `hiddenProperties`
with an empty set and adds null-safe handling in the
`withHiddenProperties()` method.

### Why are the changes needed?
Without this fix, calling `properties()` on an `EntityCombinedFileset`
instance before setting hidden properties could cause NPEs, breaking
fileset property retrieval. Adding proper initialization ensures that
the object is always in a valid state and handles missing hidden
properties gracefully.

Fixes apache#8168

### Does this PR introduce any user-facing change?
No. Valid requests behave the same. Cases where `hiddenProperties` was
not explicitly set are now safely handled without throwing exceptions.

### How was this patch tested?
* Added unit tests to verify that calling `properties()` without setting
hidden properties does not cause exceptions
* Added tests to ensure null input to `withHiddenProperties()` is
handled safely
* Added tests to verify that hidden property filtering works correctly
when properties are set
* Verified that existing functionality remains unchanged

…Properties field (apache#8238)

### What changes were proposed in this pull request?
In `EntityCombinedFileset.java`, the `properties()` method directly
called `hiddenProperties.contains()` without null checking, which could
throw a `NullPointerException` if `hiddenProperties` was not
initialized. This PR updates the code to initialize `hiddenProperties`
with an empty set and adds null-safe handling in the
`withHiddenProperties()` method.

### Why are the changes needed?
Without this fix, calling `properties()` on an `EntityCombinedFileset`
instance before setting hidden properties could cause NPEs, breaking
fileset property retrieval. Adding proper initialization ensures that
the object is always in a valid state and handles missing hidden
properties gracefully.

Fixes apache#8168

### Does this PR introduce any user-facing change?
No. Valid requests behave the same. Cases where `hiddenProperties` was
not explicitly set are now safely handled without throwing exceptions.

### How was this patch tested?
* Added unit tests to verify that calling `properties()` without setting
hidden properties does not cause exceptions
* Added tests to ensure null input to `withHiddenProperties()` is
handled safely
* Added tests to verify that hidden property filtering works correctly
when properties are set
* Verified that existing functionality remains unchanged