Don't output traits if they are empty.

[bsneed]

• Added OmitIfNull property wrapper to avoid outputting keys with values that are null.
• Modified context traits to only output if non-null.
• Added test.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 71.07%. Comparing base (59ae167) to head (62dd673).

Files with missing lines	Patch %	Lines
Sources/Segment/Utilities/OmitIfNull.swift	50.00%	8 Missing ⚠️

❌ Your patch status has failed because the patch coverage (50.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #383      +/-   ##
==========================================
- Coverage   71.12%   71.07%   -0.05%     
==========================================
  Files          48       49       +1     
  Lines        4894     4910      +16     
==========================================
+ Hits         3481     3490       +9     
- Misses       1413     1420       +7     

Files with missing lines	Coverage Δ
Sources/Segment/Types.swift	44.05% <ø> (ø)
Sources/Segment/Utilities/OmitIfNull.swift	50.00% <50.00%> (ø)

... and 1 file with indirect coverage changes

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[bsneed]

Closing this PR as it's unnecessary. Empty traits are being added server-side, not the library.