gitea: add check if pull_request webhook is missing pull info

[6543]

no idea why gitea created the webhook in such a cursed way ... jet but this fixes an 500 error out in the wild and at least make a clear error

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 26.51%. Comparing base (b0f52b9) to head (cc003ea).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
server/forge/gitea/parse.go	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4305      +/-   ##
==========================================
- Coverage   26.52%   26.51%   -0.02%     
==========================================
  Files         377      378       +1     
  Lines       27448    27500      +52     
==========================================
+ Hits         7281     7291      +10     
- Misses      19503    19543      +40     
- Partials      664      666       +2     

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

[woodpecker-bot]

Tearing down https://woodpecker-ci-woodpecker-pr-4305.surge.sh