bugfix(microservices/nats): check replyTo channel existence

[vbrvk]

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?

[x] 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?

Issue Number: #4874

What is the new behavior?

App doesn't crash for valid request

Does this PR introduce a breaking change?

[ ] Yes
[X] No

[coveralls]

Pull Request Test Coverage Report for Build c71a66d5-5fed-421a-b033-e2c93189f5bf

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 94.82%

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Totals
Change from base Build 8866f8ae-b389-4900-bbda-251e1de94805:	0.002%
Covered Lines:	4924
Relevant Lines:	5193

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💛 - Coveralls

[vbrvk]

Linter failed because of Unexpected empty arrow function @typescript-eslint/no-empty-function
Can I mark the line as ignored by linter or what should I do? 🤔