Raise a proper value error message when the metadata is empty

sdv/metadata/metadata.py

@@ -162,6 +162,11 @@ def _convert_to_single_table(self):
         return next(iter(self.tables.values()), SingleTableMetadata())
 
     def _handle_table_name(self, table_name):
+        if len(self.tables) == 0:
+            raise ValueError(
+                'The metadata object is currently empty. To populate it, please use either '
+                "'detect_from_dataframe' or 'detect_from_dataframes'."
+            )
         if table_name is None:
             if len(self.tables) == 1:
                 table_name = next(iter(self.tables))

tests/unit/metadata/test_metadata.py

@@ -692,3 +692,16 @@ def test_update_methods(self, method, args):
             # Assert
             metadata._handle_table_name.assert_called_once_with('table_name')
             mock_super_method.assert_called_once_with('table_name', *args)
+
+    def test__handle_table_name_with_empty_tables(self):
+        """Test that the proper `ValueError` is raised when there are no `tables`."""
+        # Setup
+        instance = Metadata()
+
+        # Run and Assert
+        error_msg = (
+            'The metadata object is currently empty. To populate it, please use either '
+            "'detect_from_dataframe' or 'detect_from_dataframes'."
+        )
+        with pytest.raises(ValueError, match=error_msg):
+            instance._handle_table_name(None)