Handle when some legal doc metadata fields might be present but None

[lizadaly]

fixes #1836

I tested this locally using the same casebook that Sentry flagged (it was just a bot crawling the site so it was deep on page 10 of an older casebook).

I assume it was the middle item here:

[codecov-commenter]

Codecov Report

Merging #1837 (af0c668) into develop (db0b32f) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1837   +/-   ##
========================================
  Coverage    76.96%   76.96%           
========================================
  Files           50       50           
  Lines         6563     6565    +2     
========================================
+ Hits          5051     5053    +2     
  Misses        1512     1512           

Impacted Files	Coverage Δ
web/main/models.py	75.66% <100.00%> (+0.02%)	:arrow_up:

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