SearchSettings: fix bug with empty search-text
SearchSettings: fix bug with empty search-text

When setting an empty search-text (""), the set_search_text() function
store NULL instead. Other places in the code (in SearchContext) assume
that.

This avoids an infinite loop in the SearchContext.

GNOME/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,uajain/gtksourceview