terminal-util: drop assert() on 'x'

We happily take a NULL strv as argument, equivalent to an empty one,
like most of our strv handling funcs.

src/basic/terminal-util.c

@@ -529,7 +529,6 @@ int show_menu(char **x,
               const char *grey_prefix,
               bool with_numbers) {
 
-        assert(x);
         assert(n_columns > 0);
         assert(column_width > 0);