MDEV-12851: Case with window functions query crashes server

The "is null" function performs one operation which no other Item_func
does, which is to update used tables during fix_length_and_dec().

This however can not be performed before window functions have had a
chance to resolve their order by and partition by definitions, which
happens after the initial setup_fields call. Consequently, do not call
Item_func_isnull update_used_tables during fix_length_and_dec().

There was another issue detected once the crash was resolved.
Because window functions did not implement is_null() method, we would
end up returning bad results for "is null" and "is not null" functions.
Implemented is_null() method for Item_windowfunc.

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