Remove single node cluster validation check from benchmark (#266)

Allow redis-benchmark to run against single shard clusters. 

Signed-off-by: Harkrishn Patro <harkrisp@amazon.com>

src/valkey-benchmark.c

@@ -1787,7 +1787,7 @@ int main(int argc, char **argv) {
             }
             exit(1);
         }
-        if (config.cluster_node_count <= 1) {
+        if (config.cluster_node_count == 0) {
             fprintf(stderr, "Invalid cluster: %d node(s).\n",
                     config.cluster_node_count);
             exit(1);