fix: Set billing term starts at 30 days for null values

[zachaysan]

Thanks for submitting a PR! Please check the boxes below:

• I have run pre-commit to check linting
• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

Set billing term starts at 30 days for null values of the billing starting at date. Since this function is used to show API usage and isn't involved with calculating billing amounts, it's ok to default to a rolling thirty day period for API usage reporting. Solves this Sentry error.

How did you test this code?

N/A

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 29, 2024 7:25pm
flagsmith-frontend-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 29, 2024 7:25pm
flagsmith-frontend-staging	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 29, 2024 7:25pm

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: api/app_analytics/analytics_db_service.py

Function	Unhandled Issue
get_usage_data	TypeError: unsupported operand type(s) for +: 'relativedelta' and 'NoneType' /api/v1/organisations/{... Event Count: 5

---

_{Did you find this useful? React with a 👍 or 👎}

[github-actions]

Uffizzi Preview deployment-52408 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.42%. Comparing base (9e2ffd2) to head (fb7d580).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4053   +/-   ##
=======================================
  Coverage   96.42%   96.42%           
=======================================
  Files        1146     1146           
  Lines       37386    37386           
=======================================
  Hits        36048    36048           
  Misses       1338     1338           

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