Also prevented use of empty exact matches.
Prevents issues when attempting to use exact search terms in inputs for
just search terms, and use of single " chars within search terms since
these would get auto-promoted to exacts.
For #4535
Primarily intended to show parent book for chapters when moving/copying
pages, since the default parent selector interfaces, which used the
entity-selector search endpoint, would run this popular query when no
term was present as a default backup.
For #4264
Dan Brown
esakkiraja100116