fix unit tests

document_page_reference/models/document_page.py

@@ -23,12 +23,10 @@ try:
     class Context(SandboxedEnvironment.context_class):
         def resolve_or_missing(self, key):
             res = super().resolve_or_missing(key)
-            if res is not Undefined:
+            if not isinstance(res, Undefined):
                 return res
-            try:
-                return self.parent["ref"](key)
-            except KeyError:
-                return Undefined()
+            # If the key is not found in the normal context, try to resolve it as a reference
+            return self.parent["ref"](key)
 
     class Environment(SandboxedEnvironment):
         context_class = Context