Check for description being `None` when composing PR description

[webknjaz]

Example: aio-libs/aiohttp#11772.

This is the suspect: https://github.com/sanitizers/patchback-github-app/blob/5bd473a3e3593039610c8be502c011fbe0e21306/patchback/event_handlers.py#L448C46-L448C53