Invalid paths in lcov input leads to files silently disappearing

If there is a misconfiguration that causes the file paths in lcov.info to be invalid, the files are missing from the output to coveralls with no warning or error.

Relates to https://github.com/coverallsapp/github-action/issues/103

For example in this ctest (Debug) run: https://github.com/ShrapnelDSP/ShrapnelMonorepo/runs/5343824003

# lcov input
SF:/__w/ShrapnelMonorepo/ShrapnelMonorepo/firmware/components/audio_param/include/audio_param.h

Run coverallsapp/github-action@master
  with:
    path-to-lcov: lcov.info
    base-path: test
    github-token: ***
    parallel: true
    flag-name: ctest
    coveralls-endpoint: https://coveralls.io/

# locv after processing by coveralls action
SF:test/__w/ShrapnelMonorepo/ShrapnelMonorepo/firmware/components/audio_param/include/audio_param.h\n

[info] "2022-02-26T10:17:40.263Z"  'sending this to coveralls.io: ' '{"source_files":[],"flag_name":"ctest"

Start with the action's handling of the lcov.info input, using the shown SF: path and base-path: test configuration as a reproduction. Trace how invalid paths become an empty source_files payload; done means the action reports the invalid files instead of silently sending no source files.