Add abort message if all elements of a tree are removed

[holke]

Describe your changes here:

Add an abort message that triggers if all elements of a tree are removed during adapt.
We currently do not support deleting all elements of a tree and if a user does so they should be warned. See also #1142 #1137.

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Licence

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[lukasdreyer]

The abort call was the wrong way around.

Now we only need to disable the empty local tree test