Fix UnboundLocalError in MarkItDown._convert (#1038)

Initialize `res` at the beginning of `_convert`. If the first converter raises an exception, then the `res` variable was not initialized and we got an error when checking `if res is not None`

packages/markitdown/src/markitdown/_markitdown.py

@@ -312,6 +312,7 @@ class MarkItDown:
     def _convert(
         self, local_path: str, extensions: List[Union[str, None]], **kwargs
     ) -> DocumentConverterResult:
+        res: Union[None, DocumentConverterResult] = None
         error_trace = ""
 
         # Create a copy of the page_converters list, sorted by priority.