More fixes to k4_set_ptt

https://github.com/Hamlib/Hamlib/issues/864

rigs/kenwood/k3.c

@@ -2698,11 +2698,6 @@ int k4_set_ptt(RIG *rig, vfo_t vfo, ptt_t ptt)
 
     rig_debug(RIG_DEBUG_VERBOSE, "%s called\n", __func__);
 
-    if (!ptt)
-    {
-        return -RIG_EINVAL;
-    }
-
     snprintf(cmd, sizeof(cmd), "TX0");
     if (ptt) cmd[2] = '1';
     retval = kenwood_transaction(rig, cmd, NULL, 0);