fix(tracing): Don't send a null parent_span_id from sentry_set_trace

[limbonaut]

sentry_set_trace wrote parent_span_id unconditionally, so a caller passing no parent got a stored null that serialized to "parent_span_id":null on the wire, and a caller passing an empty string got "parent_span_id":"". Other SDKs leave the key out when there is no parent, and Native already does so elsewhere: sentry_regenerate_trace never sets it, and the transaction path removes it when it is empty-ish. This guards the write so a NULL or zero-length parent omits the key, which is what a downstream SDK needs when it starts a fresh trace.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.37%. Comparing base (de1d458) to head (38267a7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2047      +/-   ##
==========================================
- Coverage   74.37%   74.37%   -0.01%     
==========================================
  Files         103      103              
  Lines       26665    26667       +2     
  Branches     4852     4853       +1     
==========================================
+ Hits        19832    19833       +1     
+ Misses       5501     5498       -3     
- Partials     1332     1336       +4     

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