fix(microservices): enable grpc option value to accept 0

[Jekiwijaya]

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?

currently the logic ignore if value grpc option is 0

Issue Number: #4816

What is the new behavior?

grpc option should be able accept 0 value

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 15cd463e-2437-4b10-9e30-ba44b65e06c7

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.629%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/microservices/client/client-grpc.ts	0	1	0.0%

Totals
Change from base Build d756523f-c85a-4bd9-827d-44fd12e77f26:	0.0%
Covered Lines:	4704
Relevant Lines:	4971

---

💛 - Coveralls

[kamilmysliwiec]

Thank you!