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PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Reflector.createDecorator<TParam>() internally stores metadata values using value ?? {}.
Because null is coalesced to {}, an explicit @Decorator(null) does not preserve the intended null value. The reflected result contradicts the decorator's TypeScript type contract.

Example:

const Metadata = Reflector.createDecorator<string | null>();

@Metadata(null)
class Test {}

// Expected: null
// Actual:   {}
new Reflector().get(Metadata, Test);

The same issue affects transform functions that intentionally return null.

Issue Number: N/A

What is the new behavior?

Only a genuinely missing value (undefined) falls back to the empty-object marker used by argument-less decorators. An explicit null (or a transform result of null) is now preserved unchanged.

This is consistent with how getAllAndOverride already treats metadata — it skips undefined but keeps null as a legitimate value.

Does this PR introduce a breaking change?

• Yes
• No

The fallback to {} is only relevant for decorators called without arguments (e.g. @Decorator()). All existing non-null usages continue to behave identically.

Other information

• Added four unit tests covering:
  1. explicit null metadata value
  2. transform returning null
  3. argument-less decorator still produces {}
  4. other falsy values (0, '', false) remain unchanged