fix(ws): filter only nil results

[CodyTseng]

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?

Issue Number: N/A

If the message handler return false, the ws-adapter will filter this result.

What is the new behavior?

Only nil results will be filtered. This behavior is the same as io-adapter.

Does this PR introduce a breaking change?

• Yes
• No

Other information

By the way, I don't think null should be filtered 🤔.

[coveralls]

Pull Request Test Coverage Report for Build 897ec688-b228-40a3-9dc1-fc6fb90f5c52

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.885%

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Totals
Change from base Build 5e72b472-ecc8-4d1e-a9fd-634e8558b976:	0.0%
Covered Lines:	6488
Relevant Lines:	6985

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💛 - Coveralls

[kamilmysliwiec]

lgtm