Add null safety to Firebase cache manager

[percula]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

This PR adds null safety to Firebase cache manager and updates dependencies to their null safe versions.

:arrow_heading_down: What is the current behavior?

:new: What is the new behavior (if this is a feature change)?

:boom: Does this PR introduce a breaking change?

:bug: Recommendations for testing

:memo: Links to relevant issues/docs

:thinking: Checklist before submitting

• [x] All projects build
• [x] Follows style guide lines (code style guide)
• [x] Relevant documentation was updated
• [x] Rebased onto current develop

[renefloor]

Thanks, I made two small changes.