Fix attempt to replicate empty fragments

* Adds method wsrep::transaction::streaming_step() so that there is a
  single place where streaming context unit counter is udpated.
  The method also checks that some data has been generated before
  attempting fragment replication.
* Emit a warning if there is an attempt to replicate a fragment and
  there is no data to replicate.

test/mock_client_state.hpp

@@ -200,6 +200,12 @@ namespace wsrep
             : mock_client_state(server_state, *this, id, mode)
             , mock_client_service(static_cast<mock_client_state&>(*this))
         { }
+
+        int after_row()
+        {
+            bytes_generated_++;
+            return wsrep::client_state::after_row();
+        }
     };
 }