Fix BuildItem with empty BuildLine reference

[SchrodingersGat]

Ref: #7137

Discovered that the BuildItem model does not get deleted when the parent BuildLine item is deleted. This can cause build allocations to remain in the database even when the parent BuildLine (or BuildLine.Build) is deleted from the database.

[netlify]

✅ Deploy Preview for inventree-web-pui-preview canceled.

Name	Link
🔨 Latest commit	e1d5d10
🔍 Latest deploy log	https://app.netlify.com/sites/inventree-web-pui-preview/deploys/663ad898ba69f200080e4cba

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.82%. Comparing base (108bd28) to head (e1d5d10).

Files	Patch %	Lines
...enTree/build/migrations/0050_auto_20240508_0138.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7178   +/-   ##
=======================================
  Coverage   83.82%   83.82%           
=======================================
  Files        1045     1047    +2     
  Lines       46274    46289   +15     
  Branches     1354     1354           
=======================================
+ Hits        38787    38801   +14     
- Misses       7169     7170    +1     
  Partials      318      318           

Flag	Coverage Δ
backend	85.44% <86.66%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.