Add @Nullable to equalTo value arg

[trask]

open-telemetry/opentelemetry-java-instrumentation#17065 (comment)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (00f950a) to head (24837ad).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8301      +/-   ##
============================================
- Coverage     90.26%   90.25%   -0.01%     
+ Complexity     7686     7685       -1     
============================================
  Files           850      850              
  Lines         23199    23199              
  Branches       2354     2354              
============================================
- Hits          20941    20939       -2     
- Misses         1531     1532       +1     
- Partials        727      728       +1     

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