[core] Handle missing link nodes when deserializing edges

Avoid uncaught errors when deserializing edges in case linked
nodes are missing.
Handle missing nodes the same way missing attributes are dealt with.

tests/test_graphIO.py

@@ -336,3 +336,15 @@ class TestImportGraphContentFromMinimalGraphData:
             assert len(graph.nodes) == 1
             assert len(graph.compatibilityNodes) == 0
 
+    def test_connectionsToMissingNodesAreDiscarded(self):
+        graph = Graph("")
+
+        with registeredNodeTypes([SimpleNode]):
+            sampleGraphContent = dedent("""
+            {
+                "SimpleNode_1": { 
+                    "nodeType": "SimpleNode", "inputs": { "input": "{NotSerializedNode.output}" } 
+                }
+            }
+            """)
+            graph._deserialize(json.loads(sampleGraphContent))