msf layer needs _pages usage fix

[atcuno]

pyright complained about this line:

https://github.com/volatilityfoundation/volatility3/blob/develop/volatility3/framework/layers/msf.py#L238

Saying self._pages could be None which would cause a backtrace since its not checked before being indexed.

So then I looked at where it is initialized and it is in the constructor:

https://github.com/volatilityfoundation/volatility3/blob/develop/volatility3/framework/layers/msf.py#L197

with a default argument of None. Then on the line right after (198), len() is called on it before checking which would break too. pyright also complained about that len():

 573   /home/rk/volatility3/volatility3/framework/layers/msf.py:198:31 - error: Argument of type "Unknown | None" cannot be assigned to parameter "obj" of type "Sized" in functio     n "len"

@ikelos I am assigning this to you since you had the commits in the history.