Start enabling empty types in pattern matching.

Remove the assumption at the start of is_useful that any suitably-long array of
wildcard patterns is useful relative the any empty vector. Instead we just
continue to recurse column-wise over the matrix.

This assumption is false in the presence of empty types.
eg. in the simplest case:

let x: ! = ...;
match x {
    // This pattern should not be considered useful by the algorithm
    _   => ...
}

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