Added due to now not being able to perform an exact search where
contains a trailing backslash.
Now all backslashes in exact terms are consided escape chars
and require escaping themselves.
Potential breaking change due to search syntax handling change.
Related to #4535.
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
