fix(microservices): fix double callback when isdisposed or err is truthy

[LhonRafaat]

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?

when isDisposed or err is truthy, the callback is invoked twice. Can cause duplicate message processing.

Issue Number: N/A

What is the new behavior?

If isDisposed or err is truthy, returns the callback, so the callback below it does not get executed.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 1a5d0d28-24a9-47a0-8ba1-e37b329f5833

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.863%

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Totals
Change from base Build e9fca389-3e6c-422f-afba-e2baae073f48:	0.0%
Covered Lines:	7473
Relevant Lines:	8316

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💛 - Coveralls

[kamilmysliwiec]

LGTM