Fix: Duplicated Android Breadcrumbs with no Mechanism

[denrase]

📜 Description

Closes #952

💚 How did you test it?

Added unit tests.

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated the docs if needed
• All tests passing
• No breaking changes

[codecov-commenter]

Codecov Report

Merging #954 (a1c89d3) into main (5a205e7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #954   +/-   ##
=======================================
  Coverage   89.81%   89.81%           
=======================================
  Files         104      104           
  Lines        3212     3212           
=======================================
  Hits         2885     2885           
  Misses        327      327           

Impacted Files	Coverage Δ
dart/lib/src/sentry_client.dart	92.53% <100.00%> (ø)

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[denrase]

@marandaneto The "CI" issue was that we would run on "Android" as a platform, and we changed the behaviour to removing breadcrumbs if there are no mechanisms, which is also true if there are no exceptions. I have set a fixed platform (iOS) now for this particular unit test file.