Handle empty url and oauth_host on login page

[anbraten]

fixes #5408

[codecov]

Codecov Report

❌ Patch coverage is 68.57143% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.50%. Comparing base (af5ae4f) to head (e2c334f).

Files with missing lines	Patch %	Lines
server/forge/setup/setup.go	0.00%	6 Missing ⚠️
server/forge/bitbucket/bitbucket.go	82.75%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5434   +/-   ##
=======================================
  Coverage   26.50%   26.50%           
=======================================
  Files         403      403           
  Lines       28817    28838   +21     
=======================================
+ Hits         7637     7644    +7     
- Misses      20485    20498   +13     
- Partials      695      696    +1     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[qwerty287]

Why should we need this? Bitbucket (difference to all other forges) is not self-hostable. It should never be necessary to skip TLS verification or to use a custom oauth host as you can't run it with self-signed certificates or in the same internal network as woodpecker.