Fixes undefined handler passing to the gRPC stream

[olegbespalov]

What?

This PR fixes a panic that happens if pass undefined (null) handler to the stream.on

E.g.

stream.on("data", undefined)

Why?

We shouldn't panic

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.86%. Comparing base (e5b00db) to head (e4c6dae).

❗ Current head e4c6dae differs from pull request most recent head de14138

Please upload reports for the commit de14138 to get more accurate results.

Files	Patch %	Lines
js/modules/k6/grpc/stream.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3779      +/-   ##
==========================================
+ Coverage   70.84%   70.86%   +0.01%     
==========================================
  Files         291      291              
  Lines       21222    21224       +2     
==========================================
+ Hits        15035    15040       +5     
+ Misses       5222     5219       -3     
  Partials      965      965              

Flag	Coverage Δ
ubuntu	70.77% <66.66%> (-0.02%)	⬇️
windows	70.70% <66.66%> (-0.01%)	⬇️

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[joanlopez]

Similarly to what I left here, this time I think we do actually need an extra check for the sake of correctness. Right? 🤔

[joanlopez]

Similarly to what I left here, this time I think we do actually need an extra check for the sake of correctness. Right? 🤔

I quickly checked that, and it returns a TypeError. And, although I still think the error should look pretty similar to the one returned when the handler is undefined, I'm gonna approve the PR because that corner case doesn't produce a panic, so we can address it later, in another PR.