fix(microservices): when postfixId is an empty string on kafka

[Auspicus]

getOptionsProp does an || with the passed value so empty strings end up being set as undefined

We may want to add a test for this since it doesn't seem like this behavior was covered.

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?

Passing postfixId: '' does not do what you'd expect and instead continues to the use the default postfixId values

What is the new behavior?

Passing postfixId: '' will now correctly unset the postfixId and disable that behavior

Does this PR introduce a breaking change?

• Yes
• No

Current behavior is a bug but users might be relying on the bug.

Other information

[coveralls]

Pull Request Test Coverage Report for Build 6c2e47bb-fd86-4b30-922b-f0dbb7761765

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 92.679%

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Totals
Change from base Build 4d97af2b-5b7e-488e-a334-ead2f9bfc8a8:	0.004%
Covered Lines:	6520
Relevant Lines:	7035

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💛 - Coveralls

[Auspicus]

Fixed by: #10549