startup: report an empty search string also when there is a modifier

Not just for +/ (a search command without a string) should nano report
an "Empty search string", but also for +c/ or +r/ or similar.

This fixes https://savannah.gnu.org/bugs/?63222.

Bug existed since version 4.4, commit 2326bf69.

src/nano.c

@@ -2454,7 +2454,7 @@ int main(int argc, char **argv)
 					searchstring = copy_of(&argv[optind][n + 1]);
 					if (argv[optind][n] == '?')
 						SET(BACKWARDS_SEARCH);
-				} else if (n == 1)
+				} else
 					statusline(ALERT, _("Empty search string"));
 				optind++;
 			} else