fix: don't create tags w/ empty name for internal zipkin spans

[mzahor]

Resolves #2595

For spans w/ span kind == "internal" and remoteEndpoint != nil, spanV2ToThrift would add a tag with empty name. This PR fixes the issue.

[yurishkuro]

Thanks!

Pull request has been modified.

[codecov]

Codecov Report

Merging #2596 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2596      +/-   ##
==========================================
- Coverage   95.32%   95.31%   -0.01%     
==========================================
  Files         208      208              
  Lines        9281     9285       +4     
==========================================
+ Hits         8847     8850       +3     
- Misses        355      356       +1     
  Partials       79       79              

Impacted Files	Coverage Δ
cmd/collector/app/zipkin/jsonv2.go	100.00% <100.00%> (ø)
pkg/config/tlscfg/cert_watcher.go	94.52% <100.00%> (+0.07%)	⬆️
plugin/storage/integration/integration.go	77.34% <0.00%> (-0.56%)	⬇️

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[yurishkuro]

Thanks!