Do not start searching if query is empty (#9333)

This avoids the problem of a search being kicked off involuntarily and
potentially using a large amount of CPU when toggling on the `Search
Ignored Files` option.

What would happen is that someone would turn the option on, we'd kick
off a search, and go through all of the files in, say, `node_modules`.
Even if no query was given.

This avoids that.

Release Notes:

- Fixed an empty search being kicked off involuntarily if no query was
typed in yet but an option was toggled.