Fix a formatError exception when parsing some Linux traces.

[vancem]

This was caused by a trace with an empty process name. Fixed to be resilient to that.

Also made the code clearer while I was in there.

[codecov-io]

Codecov Report

Merging #890 into master will decrease coverage by <.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #890      +/-   ##
==========================================
- Coverage   17.48%   17.47%   -0.01%     
==========================================
  Files         228      228              
  Lines      139263   139271       +8     
  Branches    12334    12336       +2     
==========================================
- Hits        24353    24341      -12     
- Misses     113967   113985      +18     
- Partials      943      945       +2

Flag	Coverage Δ
#2017	17.47% <33.33%> (-0.01%)	⬇️
#Debug	17.47% <33.33%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
...w/OtherSources/Linux/LinuxPerfScriptEventParser.cs	75.85% <33.33%> (-1.17%)	⬇️
src/PerfView/Utilities/FastStream.cs	45.85% <0%> (-2.54%)	⬇️

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