Scale out handler does not have enough guarding flags

Related to LP Bug #1914299 and the attempt to handle departed hooks [1] that *may* be reverted.

The reactive.mysql_innodb_cluster_handlers.scale_out function may be called when the functions it calls do not have all of their required flags set:
    create_remote_cluster_user()
    configure_instances_for_clustering()
    add_instances_to_cluster()

Leading to:

2021-02-06 22:29:08 INFO juju-log Invoking reactive handler: reactive/mysql_innodb_cluster_handlers.py:291:scale_out
2021-02-06 22:29:08 ERROR juju-log Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.8/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/.venv/lib/python3.8/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/charm/reactive/mysql_innodb_cluster_handlers.py", line 310, in scale_out
    create_remote_cluster_user()
  File "/var/lib/juju/agents/unit-mysql-innodb-cluster-2/charm/reactive/mysql_innodb_cluster_handlers.py", line 105, in create_remote_cluster_user
    for unit in cluster.all_joined_units:
AttributeError: 'NoneType' object has no attribute 'all_joined_units'

Also related is the discussion on issue/231 [2]. That endpoint_from_flag may return None.

We should audit the charm for these edge cases and guard against None from endpoint_from_flag.

A comprehensive solution for departed/broken relationships is needed.

[0] https://bugs.launchpad.net/charm-mysql-router/+bug/1914299
[1] https://review.opendev.org/c/openstack/charm-mysql-innodb-cluster/+/774033
[2] https://github.com/juju-solutions/charms.reactive/issues/231