Improve check for `Missing Private Key` alert

[jeffpaul]

Is your enhancement related to a problem? Please describe.

When the plugin is first activated and a user gets to the Apple Maps block, the fact that the MapKit credentials are not yet saved throws a Missing Private Key alert on a rather frequent basis. This is generally not super helpful and would be worth assessing how we can handle that check a bit better. Perhaps waiting until someone clicks the Confirm MapKit Credentials button and/or any other ways to ensure we're only throwing that alert when its clear a user is trying to use the Apple Maps block but is not properly configured.

See a bit more on this from https://github.com/10up/maps-block-apple/pull/196#issuecomment-1861825855

Designs

n/a

Describe alternatives you've considered

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