fix Repo form in django admin

[nora-codecov]

Purpose/Motivation

It's a common issue to have null=True or a default_value without blank=True on a field in a django model. This causes a conflict in forms, which interprets the field as required even though you intend for the value to be null or the default_value set on the field. In my case, I was trying to clear out the webhook_secret field, but my changes were blocked because the webhook_secret cannot be blank, therefore there was no way to get it back to the null value I wanted, which is allowed in the database.

Links to relevant tickets

part of https://github.com/codecov/internal-issues/issues/366

What does this PR do?

I need to be able to delete the webhook_secret, this was not allowed by the form. These changes allow me to clear out that field, fixed it on a few other fields on the model as well so that it behaves closer to how we intended.

[codecov-notifications]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.14%. Comparing base (80d64ad) to head (3ce07ae).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@               Coverage Diff                @@
##               main       #772        +/-   ##
================================================
+ Coverage   96.13000   96.14000   +0.01000     
================================================
  Files           812        812                
  Lines         18385      18395        +10     
================================================
+ Hits          17675      17685        +10     
  Misses          710        710                

Flag	Coverage Δ
unit	91.94% <100.00%> (+<0.01%)	⬆️
unit-latest-uploader	91.94% <100.00%> (+<0.01%)	⬆️

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