- Let Item::save_in_field() call set_field_to_null_with_conversions() 
  for decimal type, like this is done for the other item result types.


mysql-test/r/type_decimal.result:
  BUG#31450: Query causes error 1048: testcase
mysql-test/t/type_decimal.test:
  BUG#31450: Query causes error 1048: testcase