fix: handle None constraints

Author: AmirLayegh

src/neo4j_graphrag/experimental/components/schema.py

@@ -261,13 +261,13 @@ def validate_constraints_against_node_types(self) -> Self:
         if not self.constraints:
             return self
         for constraint in self.constraints:
-            if not constraint.get("property_name"):
+            if not constraint.property_name:
                 raise SchemaValidationError(
                     f"Constraint has no property name: {constraint}. Property name is required."
                 )
-            if constraint.get("node_type") not in self._node_type_index:
+            if constraint.node_type not in self._node_type_index:
                 raise SchemaValidationError(
-                    f"Constraint references undefined node type: {constraint.get('node_type')}"
+                    f"Constraint references undefined node type: {constraint.node_type}"
                 )
         return self