Fix obsure error if structured metadata is fetched from a NullCodec schema

Author: benjeffery

python/tests/test_highlevel.py

@@ -5485,3 +5485,9 @@ def test_array_contents(self, struct_metadata_ts, table_name):
             assert row["name"] == b"name"
             assert row["value"] == 1.0
             assert row["active"]
+
+    @pytest.mark.parametrize("table_name", metadata_tables)
+    def test_error_if_no_schema(self, table_name):
+        ts = msprime.simulate(10)
+        with pytest.raises(NotImplementedError):
+            getattr(ts, f"{table_name}_metadata")

python/tskit/metadata.py

@@ -787,6 +787,7 @@ def __init__(self, schema: Mapping[str, Any] | None) -> None:
             self.encode_row = NOOPCodec({}).encode
             self.decode_row = NOOPCodec({}).decode
             self.empty_value = b""
+            self.codec_instance = NOOPCodec({})
         else:
             try:
                 TSKITMetadataSchemaValidator.check_schema(schema)