Improve merging of str patterns

[waveform80]

When the merge phase combines Str patterns, if either is undefined or unequal lengths are found, we just ditch the entire pattern. This could be more refined, e.g.zip_longest and mark the suffix regex-optional, although that begs the question what to do about combining optional suffixes, or if such a thing is even valid. Still, there's almost certainly room for some improvement here.