Update Connector to allow creation of incomplete graph (without a Producer at root)

include/aidge/graph/Connector.hpp

@@ -83,4 +83,4 @@ class Connector {
 std::shared_ptr<GraphView> generateGraph(std::vector<Connector> ctors);
 }  // namespace Aidge
 
-#endif /* AIDGE_CORE_GRAPH_CONNECTOR_H_ */
\ No newline at end of file
+#endif /* AIDGE_CORE_GRAPH_CONNECTOR_H_ */

src/graph/Connector.cpp

@@ -41,6 +41,7 @@ std::shared_ptr<Aidge::GraphView> Aidge::generateGraph(std::vector<Connector> ct
             std::vector<std::shared_ptr<Node>> parents = nodesToAdd.back()->getParents();
             const std::set<std::shared_ptr<Node>>& alreadyAdded = graph->getNodes();
             for (std::shared_ptr<Node> parent : parents) {
+                if (!parent) continue;
                 if (alreadyAdded.find(parent) == alreadyAdded.end()) {
                     buffer.push_back(parent);
                 }
@@ -51,4 +52,4 @@ std::shared_ptr<Aidge::GraphView> Aidge::generateGraph(std::vector<Connector> ct
         buffer = {};
     }
     return graph;
-}
\ No newline at end of file
+}