There were two problems when inferring the correct field types resulting from
UNION queries.
- If the type is NULL for all corresponding fields in the UNION, the resulting 
  type would be NULL, while the type is BINARY(0) if there is just a single 
  SELECT NULL.
- If one SELECT in the UNION uses a subselect, a temporary table is created
  to represent the subselect, and the result type defaults to a STRING type,
  hiding the fact that the type was unknown(just a NULL value).
Fixed by remembering whenever a field was created from a NULL value and pass
type NULL to the type coercion if that is the case, and creating a string field
as result of UNION only if the type would otherwise be NULL.


mysql-test/r/union.result:
  Bug#32848: Test result
mysql-test/t/union.test:
  Bug#32848: Test case
sql/field.cc:
  Bug#32848: Initialization of new field
sql/field.h:
  Bug#32848: New member to record when a field was created from a NULL value.
sql/item.cc:
  Bug#32848: 
  A field created from a NULL value will submit NULL as type to the 
  type coercion procedure.
  If Item_type_holder has not inferred the correct type after processing all
  SELECTs in a UNION, a string field is created.
sql/sql_select.cc:
  Bug#32848: Recording when a field is created from a NULL value.