fix(common): reject null file input in ParseFilePipe - #17598

PR Checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other

What is the current behavior?

ParseFilePipe.thereAreNoFilesIn() only checks for undefined via isUndefined(). When the value is null instead, it's treated as if a file were present:

• The fileIsRequired check is skipped, even though fileIsRequired defaults to true.
• Any registered validator that already treats a falsy file as trivially valid (e.g. MaxFileSizeValidator.isValid() returns true when !file) also silently passes.

As a result, new ParseFilePipe().transform(null) resolves to null instead of throwing, and the failure only surfaces later as a TypeError when the handler dereferences the file.

What is the new behavior?

thereAreNoFilesIn() now uses isNil() instead of isUndefined(), so null and undefined are handled identically — both correctly trigger the "File is required" exception (or the registered validators, if fileIsRequired is false).

Two regression tests were added:

• null with fileIsRequired: true now throws BadRequestException.
• null with a validator that mimics the built-in "skip validation when no file" behavior (like MaxFileSizeValidator) now still throws, instead of silently passing.

Does this PR introduce a breaking change?

• Yes
• No

null was never valid/expected input for a required file parameter; this only makes the pipe reject it explicitly instead of silently letting it through.