R1:625d5235a5bc

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…