emacs: set query-context to nil if its "" or "*"

The queries "" and "*" are special cased in the notmuch library to
match all messages, but only if they're the entire query. They can't
be combined with other queries, such as "* AND foo", in which case
they "leak" down to the Xapian query parser.

Notmuch show and tree buffers inadvertently combine the thread query
with said special queries, causing incorrect collapsing of
messages. Handle the special queries specially. We already do a
similar thing in notmuch-search-filter.