Allow settings value to be nullified

[SchrodingersGat]

• Settings values should be able to be "nulled" via UI and API
• Allow "null" values on the field (to prevent API errors)
• Convert any null values to empty string

[netlify]

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

Name	Link
🔨 Latest commit	8196d33
🔍 Latest deploy log	https://app.netlify.com/sites/inventree-web-pui-preview/deploys/674a8d910f36e80008c517ad

[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.

Project coverage is 84.59%. Comparing base (20d862e) to head (8196d33).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/backend/InvenTree/common/serializers.py	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8596      +/-   ##
==========================================
- Coverage   84.60%   84.59%   -0.02%     
==========================================
  Files        1189     1189              
  Lines       54264    54279      +15     
  Branches     2050     2051       +1     
==========================================
+ Hits        45910    45915       +5     
- Misses       7846     7857      +11     
+ Partials      508      507       -1     

Flag	Coverage Δ
backend	86.05% <77.77%> (-0.01%)	⬇️
pui	68.87% <100.00%> (-0.05%)	⬇️

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