fix: Check if the workload is accessible before running hook

[DanielArndt]

This ensures that pebble is accessible before running the hook, otherwise we could get a pebble error when trying to do something like access the certificate in the workload.

This issue is currently causing plenty of integration test failures. This change appears to fix that.

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