fix: Targets to fail gracefully when schema message is missing the `p…

…roperties` key (#1287)

Closes #1145

singer_sdk/target_base.py

@@ -332,6 +332,7 @@ def _process_schema_message(self, message_dict: dict) -> None:
             message_dict: The newly received schema message.
         """
         self._assert_line_requires(message_dict, requires={"stream", "schema"})
+        self._assert_line_requires(message_dict["schema"], requires={"properties"})
 
         stream_name = message_dict["stream"]
         schema = message_dict["schema"]