docs only PRs fail code coverage CI check

[JanProvaznik]

#12805
#12857

Code Coverage failed
[3 errors / 0 warnings](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1217301)

Annotations
 [Check failure on line 9 in Build log](https://github.com/dotnet/msbuild/pull/12857/files#annotation_42398343450) 

@azure-pipelines
azure-pipelines
/ msbuild-pr (Code Coverage)
Build log #L9

Publishing build artifacts failed with an error: Not found PathtoPublish: D:\a\1\s\artifacts\CoverageResults\merged.coverage
 [Check failure on line 11 in Build log](https://github.com/dotnet/msbuild/pull/12857/files#annotation_42398343487) 

@azure-pipelines
azure-pipelines
/ msbuild-pr (Code Coverage)
Build log #L11

Artifact LinuxCoreCoverage was not found for build 1217301.
 [Check failure on line 9 in Build log](https://github.com/dotnet/msbuild/pull/12857/files#annotation_42398343497) 

@azure-pipelines
azure-pipelines
/ msbuild-pr (Code Coverage)
Build log #L9

Publishing build artifacts failed with an error: Not found PathtoPublish: D:\a\1\s\artifacts\CoverageResults\merged.cobertura.xml

This is probably because testing is skipped on docs only PRs, in that case code coverage should also be skipped