HPCC-20491 Implement emptyaction in Thor

Signed-off-by: Jake Smith <jake.smith@lexisnexisrisk.com>

thorlcr/master/thactivitymaster.cpp

@@ -157,6 +157,7 @@ public:
             case TAKhttp_rowdataset:
             case TAKdistributed:
             case TAKtrace:
+            case TAKemptyaction:
                 ret = new CMasterActivity(this);
                 break;
             case TAKskipcatch:

thorlcr/slave/slave.cpp

@@ -670,6 +670,9 @@ public:
             case TAKsimpleaction:
                 ret = createNullSlave(this);
                 break;
+            case TAKemptyaction:
+                ret = createNullSinkSlave(this);
+                break;
             case TAKtopn:
                 if (queryGrouped())
                     ret = createGroupedTopNSlave(this);