fix(plugin): Fix nullable enum not having a proper enum type

[Spissable]

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• 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?

Currently when having a nullable enum SomeEnum | null, the generated output from the plugin is missing the enum attribute. It looks like this in the unit test:

...
nullableEnumValue: { required: true, nullable: true }
...

This would further down the line crash the application start with this error:

swagger/lib/services/schema-object-factory.ts

Lines 330 to 332 in 6555285

	throw new Error(
	`A circular dependency has been detected (property key: "${key}"). Please, make sure that each side of a bidirectional relationships are using lazy resolvers ("type: () => ClassType").`
	);

My usual workaround was to manually annotate these cases, however, I implemented a fix with this PR for the plugin.

What is the new behavior?

With this fix, the generated output look as expected:

...
nullableEnumValue: { required: true, nullable: true, enum: OneValueEnum }
...

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

LGTM