Fix null tags.

[SebastianStehle]

Fixes #565

Changes

• Convert null tags to correct attribute value.
• Fix tests, which were not using the correct activities.
• Fix an edge case where an invalid activity would throw exceptions in the exporter.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: SebastianStehle / name: Sebastian Stehle (a10890e)

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.

[SebastianStehle]

Please reopen

[utpilla]

Please resolve the merge conflicts.

[SebastianStehle]

Of course, have not seen that.

[SebastianStehle]

Conflicts are solved.

[codecov]

Codecov Report

Merging #566 (a976ccf) into main (bf01da1) will increase coverage by 1.05%.
The diff coverage is 50.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
+ Coverage   76.70%   77.76%   +1.05%     
==========================================
  Files         170      170              
  Lines        5161     5163       +2     
==========================================
+ Hits         3959     4015      +56     
+ Misses       1202     1148      -54     

Impacted Files	Coverage Δ
...r.Stackdriver/Implementation/ActivityExtensions.cs	71.05% <0.00%> (+71.05%)	⬆️
...y.Exporter.Stackdriver/StackdriverTraceExporter.cs	67.50% <100.00%> (+3.39%)	⬆️

[utpilla]

Please fix the failing CI task as well.

[utpilla]

@SergeyKanzhelev Could you please review this PR?

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[SebastianStehle]

I think it is a critical bug and it would be great if it can be merged asap

[utpilla]

@SebastianStehle Please add a CHANGELOG entry. After you have done that, if you would like a new version for the package released, please follow the steps mentioned here: https://github.com/open-telemetry/opentelemetry-dotnet-contrib/blob/main/CONTRIBUTING.md#how-to-request-for-release-of-package