Fix T86710: Crash When Adding Node Group.

When adding a node group there can be no inputs in the input map that
was triggering an assert.

source/blender/compositor/intern/COM_NodeOperationBuilder.cc

@@ -291,7 +291,7 @@ void NodeOperationBuilder::add_operation_input_constants()
   }
   for (Inputs::const_iterator it = pending_inputs.begin(); it != pending_inputs.end(); ++it) {
     NodeOperationInput *input = *it;
-    add_input_constant_value(input, m_input_map.lookup(input));
+    add_input_constant_value(input, m_input_map.lookup_default(input, nullptr));
   }
 }