This checking helper is frequently used in provider tests for data
sources, as a shorthand to verify that an attribute of the data source
matches with the corresponding attribute on a managed resource.
Since we now leave empty collections null in more cases, this function is
sometimes effectively asked to verify that a given attribute is _unset_
in both the data source and the resource, so here we slightly adjust the
definition of the check to consider two nulls to be equal to one another,
which at this layer manifests as the keys not being present in the state
attributes map at all.
This check function didn't previously have tests, so this commit also adds
a basic suite of tests, including coverage for the new behavior.
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