Allow soft hyphens in languages without patterns

This patch sets up an "empty" hyphenator, which it uses by default
for locales in which there is no hyphenation pattern data. This has
the effect of enabling soft hyphens (U+00AD), which were otherwise
disabled, because the "no-hyphen" code path didn't consider them.

Bug: 19605972
Change-Id: I4dcb95cee8edc48495f7c38736f5abf26fa04935