nixos/users-groups: Add assert on null shells

I recently set up a new machine with tmpfs-on-root
and switched to mutableUsers = false as part of that.
I missed that an existing user with `shell = null`
was no longer valid as part of this change.
(`shell = null` is still useful for `mutableUsers = true`.)

Add an assertion to prevent future issues.

Also fix a typo in an option name in a comment,
this confused me slightly
(I originally planned to extend that existing assertion.)

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