ENUM fields internally store their values as integers and may use integer
values as indexes to their values. Invalid values are mapped to zero value.
When storing an empty string the ENUM field fails to find an appropriate value
and tries to convert the provided string to integer. The conversion also
fails and error is returned even if the thd->count_cuted_fields is set to
CHECK_FIELD_IGNORE. This makes the range optimizer wrongly decide that an
impossible range is present.

Now the Field_enum::store() returns error while storing an empty string only
if the thd->count_cuted_fields isn't set to CHECK_FIELD_IGNORE.


sql/field.cc:
  Bug#28729: Field_enum wrongly reported an error while storing an empty string.
  Now the Field_enum::store() returns error while storing an empty string only
  if the thd->count_cuted_fields isn't set to CHECK_FIELD_IGNORE.
mysql-test/r/type_enum.result:
  Added a test case for the bug#28729: Field_enum wrongly reported an error
  while storing an empty string.
mysql-test/t/type_enum.test:
  Added a test case for the bug#28729: Field_enum wrongly reported an error
  while storing an empty string.