allow null lists in extend (SYN-10502)

[OCBender]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.73%. Comparing base (c04d2ef) to head (c2f97e7).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4886      +/-   ##
==========================================
- Coverage   97.73%   97.73%   -0.01%     
==========================================
  Files         298      298              
  Lines       62998    62998              
==========================================
- Hits        61574    61573       -1     
- Misses       1424     1425       +1     

Flag	Coverage Δ
linux	97.67% <100.00%> (-0.01%)	⬇️
linux_replay	93.46% <100.00%> (ø)

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