[DPE-2876] Fix on fetching local databag (when relation is broken)

[juditnovak]

Version 23

• Fixing exceptions raised by deleting non-existing fields
  • NOTE: partially internal garbage, thus we only add a debug() message on it,
  • Issue https://github.com/canonical/data-platform-libs/issues/110 opened , later priority
  • Problem revealed by @carlcsaposs-canonical testing MySQL Router: https://github.com/canonical/mysql-router-k8s-operator/pull/152#issuecomment-1787194530
• Fixing exceptions on get_relation() whenrelation.app` is not there
  • Reason: local databag still has to be available
  • Problem revealed by @carlcsaposs-canonical testing MySQL Router: https://github.com/canonical/mysql-router-k8s-operator/pull/152#issuecomment-1786997913