bsddialog(1): Improve --hline (help subtitle)

Improvement for bsdconfig(8): avoid to draw delimiters (or spaces) on
the bottom line if the argument of --hline is an empty string.

contrib/bsddialog/bsddialog.c

@@ -454,7 +454,8 @@ int main(int argc, char *argv[argc])
 			conf.key.f1_file = optarg;
 			break;
 		case HLINE:
-			conf.bottomtitle = optarg;
+			if (strlen(optarg) > 0)
+				conf.bottomtitle = optarg;
 			break;
 		case HMSG:
 			conf.key.f1_message = optarg;