Allow setting AzureName if it is currently blank

[theunrepentantgeek]

Describe the current behavior

Currently our validation blocks any change to AzureName because this can't be changed in Azure.

However, this also blocks the case of setting AzureName when it's empty (which can occur if the webhooks fail).

Describe the improvement

Allow AzureName to be changed if it's currently empty, allowing users to self-correct if something goes wrong (see #4285 for an example where this would have been useful).

Additional context

We'll probably need to clear the annotation with the resource ID as well, as that will need to be regenerated.