`Stylist` contains separate hashmaps for important and normal declarations,
but typically a given block only contains declarations of one importance.

Before this commit, there is an optimization where
a `PropertyDeclarationBlock` is only inserted in the corresponding map
if it has a non-zero number of declaration of a given importance.

With CSSOM, `PropertyDeclarationBlock` is gonna have interior mutability:
the importance (priority) of a declaration could change.
This optimization would become incorrect when the block is missing
in a hashmap where it should be.

This commits removes the original optimization, and replaces it with
a slightly weaker one: if a block doesn’t have any declaration
with the importance we’re cascading for, skip selector matching.