refactor(platform): more strict null checks

[viceice]

Changes

• strict null checks for bitbucket-server and gitea

Context

• https://github.com/renovatebot/renovate/issues/7154

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 32.27.5 🎉

The release is available on:

• GitHub release
• 32.27.5

Your semantic-release bot 📦🚀