add check for no inputs in insert_node (#1170)

Author: jmitrevs

hls4ml/model/graph.py

@@ -494,7 +494,7 @@ def insert_node(self, node, before=None, input_idx=0):
                 next_nodes.append(x)
 
         if before is None:
-            next_node = next((x for x in self.graph.values() if x.inputs[0] in prev_node.outputs), None)
+            next_node = next((x for x in self.graph.values() if x.inputs and x.inputs[0] in prev_node.outputs), None)
         else:
             if before not in next_nodes:
                 raise Exception(