Fix mypy false positive.

As reported in python/mypy#12682, mypy didn't recognise `bool` as a
`TypeGuard[T]` for `Optional[T]`. However it doesn't flag using `None`
as a typeguard in `filter` which behaves exactly the same.

* sphinx_polyversion/git.py

Author: real-yfprojects

sphinx_polyversion/git.py

@@ -535,4 +535,4 @@ async def handle(ref: GitRef) -> GitRef | None:
         async for ref in _get_all_refs(root):
             tasks.append(handle(ref))
 
-        return tuple(filter(bool, await asyncio.gather(*tasks)))
+        return tuple(filter(None, await asyncio.gather(*tasks)))