use empty consistently instead of size == 0 checks (#1126)

src/ngraph/graph_util.cpp

@@ -112,7 +112,7 @@ void ngraph::replace_node(std::shared_ptr<Node> target, std::shared_ptr<Node> re
         throw ngraph_error("Result nodes cannot be replaced.");
     }
 
-    if (target->get_users().size() == 0)
+    if (target->get_users().empty())
     {
         throw ngraph_error("replacing an unreachable node");
     }

src/ngraph/runtime/cpu/pass/cpu_concat_inputs.cpp

@@ -116,7 +116,7 @@ void ngraph::runtime::cpu::pass::ConcatInputs::concat_lstm_inputs()
                 {
                     NGRAPH_DEBUG << "Replacing 1st output Lstm node " << goe_node->get_name()
                                  << " with " << lstm_ht_out->get_name();
-                    if (goe_node->get_users().size() > 0)
+                    if (!goe_node->get_users().empty())
                     {
                         ngraph::replace_node(goe_node, lstm_ht_out);
                     }

src/ngraph/runtime/cpu/pass/cpu_rnn_fusion.cpp

@@ -785,7 +785,7 @@ void ngraph::runtime::cpu::pass::MultiLayerRNNFusion::construct_multi_layer_rnn_
         for (auto& rnn_goes : rnn_node->get_users())
         {
             NGRAPH_DEBUG << "rnn_goes: " << rnn_goes->get_name();
-            if (rnn_goes->get_users().size() == 0)
+            if (rnn_goes->get_users().empty())
             {
                 continue;
             }