src: add nullptr handling for NativeKeyObject

[wooffie]

Fixes: #56899

[nodejs-github-bot]

Review requested:

• @nodejs/crypto

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.16%. Comparing base (793c793) to head (f3212f6).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/crypto/crypto_keys.cc	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #56900      +/-   ##
==========================================
- Coverage   89.17%   89.16%   -0.02%     
==========================================
  Files         665      665              
  Lines      192602   192603       +1     
  Branches    37058    37055       -3     
==========================================
- Hits       171748   171729      -19     
- Misses      13664    13671       +7     
- Partials     7190     7203      +13     

Files with missing lines	Coverage Δ
src/crypto/crypto_keys.cc	72.13% <0.00%> (-0.10%)	⬇️

... and 24 files with indirect coverage changes