Improve url detection in mod pagespeed check #19595

Description:

fixes #12024

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

@bx80 generally you can merge PRs on approval unless there's a reason to leave it for the author

Seems the installation was broken due to this change. Common::getPiwikUrl uses the database, which is not available during install. Added a fallback to the method used before the change...