fix(text_editor.py): allow None as a valid expected_hash for new files to improve error handling

Author: tumf

src/mcp_text_editor/text_editor.py

@@ -218,7 +218,7 @@ async def edit_file_contents(
         self._validate_file_path(file_path)
         try:
             if not os.path.exists(file_path):
-                if expected_hash != "":  # Only allow empty hash for new files
+                if expected_hash not in ["", None]:  # Allow null hash
                     return {
                         "result": "error",
                         "reason": "File not found and non-empty hash provided",