Handle null input in StringEncoder

[laurit]

See open-telemetry/opentelemetry-java-instrumentation#18104

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (f669d31) to head (663330c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8312      +/-   ##
============================================
- Coverage     90.26%   90.25%   -0.01%     
- Complexity     7688     7691       +3     
============================================
  Files           850      850              
  Lines         23198    23203       +5     
  Branches       2354     2356       +2     
============================================
+ Hits          20940    20943       +3     
- Misses         1531     1532       +1     
- Partials        727      728       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.