Fix WalltimeHandler crash on incomplete OUTCAR + new test

[janosh]

closes materialsproject/pymatgen#2251

[codecov-commenter]

Codecov Report

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.97%. Comparing base (a305609) to head (93d1ca1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/custodian/vasp/handlers.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #412      +/-   ##
==========================================
- Coverage   53.02%   52.97%   -0.05%     
==========================================
  Files          39       39              
  Lines        3508     3511       +3     
==========================================
  Hits         1860     1860              
- Misses       1648     1651       +3     

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[janosh]

@shyuep could you take a look? also @esoteric-ephemera curious if you've seen this error before and if so, how you worked around it

[shyuep]

I am fine with this. Merged.