systemActions: Filter out empty (folded) terms

We split the search string into words using whitespace, while
GLib.tokenize_and_fold() splits on any non-alphanumeric characters.

That is, a valid search term like ',' will be tokenized as [], so
the original non-empty terms may get mapped to an empty array.

And as [].every() returns true for any condition[0], we end up
matching *all* system actions in that case. We want the exact
opposite and not return any results, so handle that case explicitly.

[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3169