NPE guard in AgentAttributeSender.removeAttribute()

[jtduffy]

Resolves #1749

Add an NPE check prior to calling .remove() on the return value from getAttributeMap().

This corrects the issue where NewRelicApiImplementation.setUserId() was throwing an exception when called outside of a transaction.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 70.81%. Comparing base (624ff4c) to head (b819262).

Files	Patch %	Lines
...ewrelic/agent/attributes/AgentAttributeSender.java	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1762      +/-   ##
============================================
- Coverage     70.83%   70.81%   -0.02%     
  Complexity     9931     9931              
============================================
  Files           825      825              
  Lines         39820    39823       +3     
  Branches       6034     6035       +1     
============================================
- Hits          28206    28201       -5     
- Misses         8891     8896       +5     
- Partials       2723     2726       +3     

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