fix: initialize res to None to prevent UnboundLocalError

Fixes issue #81 - UnboundLocalError when no converter accepts the file.

The variable res was only set inside the 'if _accepts:' block, but was
checked later outside of it. When no converter accepts the file,
res would be undefined, causing UnboundLocalError.

Author: SparkLabScout

packages/markitdown/src/markitdown/_markitdown.py

@@ -590,6 +590,7 @@ def _convert(
 
                 # Check if the converter will accept the file, and if so, try to convert it
                 _accepts = False
+                res = None
                 try:
                     _accepts = converter.accepts(file_stream, stream_info, **_kwargs)
                 except NotImplementedError: