Fix for part serializer

[SchrodingersGat]

• Allow "revision" to accept null values
• Matches internal field

[netlify]

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Name	Link
🔨 Latest commit	8b15fb8
🔍 Latest deploy log	https://app.netlify.com/sites/inventree-web-pui-preview/deploys/66d82ded38a9460008971223

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.84%. Comparing base (029df83) to head (8b15fb8).
Report is 304 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8069   +/-   ##
=======================================
  Coverage   83.84%   83.84%           
=======================================
  Files        1142     1142           
  Lines       50979    50979           
  Branches     1785     1785           
=======================================
  Hits        42745    42745           
  Misses       7784     7784           
  Partials      450      450           

Flag	Coverage Δ
backend	85.71% <100.00%> (ø)

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