HLLADD: propagate write when only variable name is given.

The following form is given:

    HLLADD myhll

No element is provided in the above case so if 'myhll' var does not
exist the result is to just create an empty HLL structure, and no update
will be performed on the registers.

In this case, the DB should still be set dirty and the command
propagated.

src/hyperloglog.c

@@ -440,6 +440,7 @@ void hllAddCommand(redisClient *c) {
          * is guaranteed to return bytes initialized to zero. */
         o = createObject(REDIS_STRING,sdsnewlen(NULL,REDIS_HLL_SIZE));
         dbAdd(c->db,c->argv[1],o);
+        updated++;
     } else {
         /* Key exists, check type */
         if (checkType(c,o,REDIS_STRING))