D5409: remotefilelog: accepting a None node to cmp

[Yuya Nishihara]

>   In context.py, basefilectx.cmp explicitly calls it with None, so it has to be
>   supported. Specifically, this breaks "hg absorb -i" currently.

IIUC, `self._filenode` should never be `None` if the given `fctx._filenode`
is `None`. If `None` were passed down to the filelog layer, exception would
be raised.

>          returns True if text is different than what is stored.
>          """
>  
> -        if node == nullid:
> +        if not node or node == nullid:
>              return True

Are we sure that the working-directory data is different from the given text?