feat: clear cached flags when hasFeatureFlags is false

[ioannisj]

:bulb: Motivation and Context

Clear cached feature flags when remote config responds with hasFeatureFlags false

:green_heart: How did you test it?

:pencil: Checklist

• [x] I reviewed the submitted code.
• [x] I added tests to verify the changes.
• [ ] I updated the docs if needed.
• [x] No breaking change or entry added to the changelog.

[marandaneto]

always tag @PostHog/team-feature-flags team for feature flag reviews as well.