the data in the system file.
The first call to C<get_next_case> after C<open> has been called retrieves
the first case in the system file. Each subsequent call retrieves the next
-case. If there are no more cases to be read, the function returns undef.
+case. If there are no more cases to be read, the function returns an empty
+list.
If the case contains system missing values, these values are set to the
empty string.