[TD-5978]<fix>: taosdemo set only one custom field. (#7301)

src/kit/taosdemo/taosdemo.c

@@ -1010,6 +1010,7 @@ static void parse_args(int argc, char *argv[], SArguments *arguments) {
                     exit(EXIT_FAILURE);
                 }
                 arguments->datatype[0] = argv[i];
+                arguments->datatype[1] = NULL;
             } else {
                 // more than one col
                 int index = 0;