Fix files possibly not found in 7z archives

Some archives have an empty "Attributes" field, i.e. `Attributes = `.

But because we the line is strip, it became `Attributes =` and the
`elif` branch was never taken, resulting with `largest_file` being
`None` after processing.

backend/utils/archive_7zip.py

@@ -42,7 +42,7 @@ def process_file_7z(
         current_size = 0
 
         for line in lines:
-            line = line.strip()
+            line = line.lstrip()
             if line.startswith("Path = "):
                 current_file = line.split(" = ")[1].strip()
             elif line.startswith("Size = "):