fix: go into blocked status when database relation is removed

[dariofaccin]

Description

This PR aims to fix #19. When database relation is removed, move into Blocked status.
As agreed, new integration tests are not executed until canonical/mongodb-k8s-operator#218 will be fixed.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library