cptbox: make sys_utimensat emulation more accurate

[quantum5]

This handles the case where times is NULL, which is valid and should not
generate EFAULT.

[codecov-commenter]

Codecov Report

Merging #927 (4af61c7) into master (2ea839a) will decrease coverage by 0.11%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master     #927      +/-   ##
==========================================
- Coverage   86.61%   86.49%   -0.12%     
==========================================
  Files         138      138              
  Lines        4670     4673       +3     
==========================================
- Hits         4045     4042       -3     
- Misses        625      631       +6     

Impacted Files	Coverage Δ
dmoj/executors/compiled_executor.py	92.76% <61.53%> (+0.14%)	⬆️
dmoj/cptbox/tracer.py	83.27% <0.00%> (-1.82%)	⬇️
dmoj/result.py	89.33% <0.00%> (-1.34%)	⬇️

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[Xyene]

LGTM