Remove unnecessary calls to _update_node_cache

- Apply suggestions from code review

Co-authored-by: Chris Markiewicz <effigies@gmail.com>

Author: HippocampusGirl

nipype/pipeline/engine/workflows.py

@@ -273,7 +273,8 @@ def connect(self, *args, **kwargs):
                 "(%s, %s): new edge data: %s", srcnode, destnode, str(edge_data)
             )
 
-        self._update_node_cache()
+        if newnodes:
+            self._update_node_cache()
 
     def disconnect(self, *args):
         """Disconnect nodes
@@ -320,8 +321,6 @@ def disconnect(self, *args):
             else:
                 self._graph.add_edges_from([(srcnode, dstnode, edge_data)])
 
-        self._update_node_cache()
-
     def add_nodes(self, nodes):
         """ Add nodes to a workflow