[Bug] Check before calling schedule_pricing_update

[SchrodingersGat]

Ref: #7807 (comment)

[netlify]

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

Name	Link
🔨 Latest commit	5c80354
🔍 Latest deploy log	https://app.netlify.com/sites/inventree-web-pui-preview/deploys/66bc324d9baec30008db4844

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 83.50%. Comparing base (2244f5f) to head (5c80354).
Report is 318 commits behind head on master.

Files with missing lines	Patch %	Lines
src/backend/InvenTree/part/models.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7871      +/-   ##
==========================================
- Coverage   83.50%   83.50%   -0.01%     
==========================================
  Files        1128     1128              
  Lines       50264    50269       +5     
  Branches     1733     1733              
==========================================
+ Hits        41973    41976       +3     
- Misses       7833     7835       +2     
  Partials      458      458              

Flag	Coverage Δ
backend	85.21% <60.00%> (-0.01%)	⬇️

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[github-actions]

💚 All backports created successfully

Status	Branch	Result
✅	0.16.x

Questions ?

Please refer to the Backport tool documentation and see the Github Action logs for details

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ Part.DoesNotExist: Part matching query does not exist. /api/part/category/{pk}/ View Issue

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