Prevent typeahead sources from querying against empty tokens

Summary:
Certain unusual queries, like [-], could tokenize into a list which included the empty string.

This would then convert into a query for ... LIKE "%" which just joins the entire table.

Instead: tokenize smarter; never return the empty token; add some test cases.

Test Plan: Ran unit tests. Queried for [[blah blah]], saw a reasonable query come out the other end.

Reviewers: chad

Reviewed By: chad

Subscribers: 20after4

Differential Revision: https://secure.phabricator.com/D16888