fix: Remove ContextLine without source code available

[bruno-garcia]

ContextLine is expected to be used when source is available (e.g within the PDB or SourceLink).

It's specially less than useful with async code:

The function name and module are correctly extracted but ContextLine is generated code.

[codecov-io]

Codecov Report

Merging #223 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   88.85%   88.94%   +0.09%     
==========================================
  Files          81       81              
  Lines        2018     2017       -1     
==========================================
+ Hits         1793     1794       +1     
+ Misses        225      223       -2

Impacted Files	Coverage Δ
...rc/Sentry/Extensibility/SentryStackTraceFactory.cs	94.93% <ø> (-0.07%)	⬇️
src/Sentry/Internal/BackgroundWorker.cs	95.9% <0%> (+1.63%)	⬆️

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