fix(agent): only try to add exception if it exists

[zsistla]

If we have a NULL function returnvalue, only try to add an exception if it exists.
Fibers return a null return value on exiting when they are suspended and never resume, but this does not create an exception or error condition in PHP.

[newrelic-php-agent-bot]

Test Suite	Status	Result
Multiverse	❌	16/18 passing
SOAK	❌	139/146 passing

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.46%. Comparing base (41ea1dd) to head (53b9a48).

Additional details and impacted files

@@               Coverage Diff               @@
##           feat/fibers    #1235      +/-   ##
===============================================
- Coverage        80.46%   80.46%   -0.01%     
===============================================
  Files              191      191              
  Lines            28308    28309       +1     
===============================================
  Hits             22779    22779              
- Misses            5529     5530       +1     

Flag	Coverage Δ
agent-for-php-7.2	80.26% <ø> (ø)
agent-for-php-7.3	80.28% <ø> (ø)
agent-for-php-7.4	80.15% <ø> (ø)
agent-for-php-8.0	79.72% <100.00%> (-0.01%)	⬇️
agent-for-php-8.1	80.04% <100.00%> (-0.01%)	⬇️
agent-for-php-8.2	79.64% <100.00%> (-0.01%)	⬇️
agent-for-php-8.3	?
agent-for-php-8.4	79.74% <100.00%> (+<0.01%)	⬆️
agent-for-php-8.5	79.05% <100.00%> (-0.01%)	⬇️

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