## Why make this change?

- As per the JSON config schema, not all properties in the config file
are mandatory.
- With #1402, in versions 0.8.49-0.8.52 those properties that are not
explicitly set, were initialized with defaults before writing to the
file system.
- However, config files generated using version 0.7.6 or lesser, may not
have these properties initialized to their defaults when writing to the
file system. Using config files generated with <= 0.7.6, to start engine
with 0.8.49 would therefore lead to null reference exceptions when the
optional properties are dereferenced.
- With PR #1684 the optional properties were initialized with their
default values to avoid the null reference, any config file that is
created or modified using the CLI results in a fully expanded file. This
is not a problem when a single config is used but leads to undesired
behavior when using the merge configuration file feature. A fully
expanded higher precedence config file when merged with a base config
file will override the values set for the optional properties in the
base file with their default values if those properties were not
explicitly defined in the higher precedence config file to begin with.
In order to retain the base config file values for the optional
properties, the higher precedence file would have to define every single
optional value exactly the same as in the base file if they desire those
to not be overridden. That defeats the purpose of providing the higher
precedence file which should ideally only have those properties that
need to be overriden. For more details with examples, please refer to
#1737.

- This PR fixes this problem by allowing optional properties to be null
and ensures there are no null reference exceptions as well when starting
engine with configs that don't have optional properties.

## What is this change?
- Modify object model constructors to accept nullable arguments for
optional properties - represented as fields in the record types. The
scalar optional properties have not been made nullable yet, will have a
follow up PR for those.
- All references to optional properties check for nulls
- During merge of config files, null + non-null values will pick up
non-null values. After the merge, if the value is still null, they are
considered as default values. To easily get default values, added some
properties to `RuntimeConfig`.
- Default value of Host.Mode is `Production`. Keep it that way. Default
of GraphQL.AllowIntrospection is true currently - will have another PR
to determine it based on host mode.

## How was this tested?
- Removed optional properties from config and ensured engine could be
started without them.
- [X] Unit Tests for deserialization and serialization of nullable
optional properties see `RuntimeConfigLoaderJsonDeserializerTests`
- [X] In the merge configuration tests, in `TestHelper`, modify base
config to contain a non-default value, env based config to not specify
that value and confirm the merged config file has the base-config value.

## Sample Request(s)

- Before fix, remove `runtime` section from config, run dab start with
0.8.49 leads to NullReferenceException.
- After the fix, starting engine is successful even when `runtime`
section is absent in the config.

// IN THE BASE config file
MISSING = use default
{ empty } = use default

// IN THE OVERRIDDEN config file
MISSING = use base
{ empty } = use base

Examples:
// all subproperties are missing
base: "user": { "first": "jerry", "last": "nixon" }
override: "user": { empty }
merged: "user": { "first": "jerry", "last": "nixon" }

// some sub properties are missing
base: "user": { "first": "jerry", "last": "nixon" }
override: "user": { "first": "jerry" }
merged: "user": { "first": "jerry", "last": "nixon" }

// parent object property is missing altogether
base: "user": { "first": "jerry", "last": "nixon" }
override: "user": MISSING (not specified in override)
merged: "user": { "first": "jerry", "last": "nixon" }

## TO DO for 0.10-rc
- A followup PR to make scalar optional properties nullable too.
- A followup PR that makes specifying an explicit NULL imply Default
values