fix problem with download files with zero size

relates to issue #80from mergin-plugin

Author: PachKosti

mergin/client.py

@@ -1026,6 +1026,14 @@ def _download_file(self, project_path, file, directory, parallel=True, diff_only
         file_dir = os.path.dirname(os.path.normpath(os.path.join(directory, file['path'])))
         basename = os.path.basename(file['diff']['path']) if diff_only else os.path.basename(file['path'])
 
+        if file['size'] == 0:
+            directory = os.path.abspath(file_dir)
+            os.makedirs(directory, exist_ok=True)
+            print(os.path.join(file_dir, basename))
+            with open(os.path.join(file_dir, basename), 'w'):
+                pass
+            return file["path"]
+
         def download_file_part(part):
             """Callback to get a part of file using request to server with Range header."""
             start = part * (1 + CHUNK_SIZE)