Fix #85 issue

[FAbrickA]

Issue: #85

If you leave empty path in route like that:

router = APIRouter(prefix="/api")

@cbv(router)
class CBV:
    @router.get("")
    def root(self) -> str:
        return "hello"

you will get error:

fastapi.exceptions.FastAPIError: Prefix and path cannot be both empty

Here I fix it.
I know that this solution may look like a crutch. However, after spending many hours I was unable to find a better solution