Add null check on RSA key checks

[samuel40791765]

Was running into segfaults with this with some internal builds switching over to AWS-LC.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.89%. Comparing base (4de8906) to head (698e1f5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2727      +/-   ##
==========================================
- Coverage   78.89%   78.89%   -0.01%     
==========================================
  Files         669      669              
  Lines      114327   114329       +2     
  Branches    16076    16076              
==========================================
- Hits        90202    90196       -6     
- Misses      23350    23356       +6     
- Partials      775      777       +2     

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