IFNULL(inet6_not_null_expr, 'foo') erroneously set its nullability to NOT NULL.
Fix:
- Moving the line "maybe_null= args[1]->maybe_null" before the call
of fix_length_and_dec2(), so the call of Type_handler method
Item_hybrid_func_fix_attributes() can reset it when desired.
- Fixing Type_handler_inet6::Item_hybrid_func_fix_attributes()
to ignore args[0] when detecting nullability of IFNULL().
6a7e646df3