We have "set" variables, which can accept empty values
(like sql_mode), and which can not (like log_output). The problem
was that the code does not distinguish them and allow empty
values for every set variable.

The fix is to introduce an attribute of a set variable telling
whether it can accept empty values.


mysql-test/r/variables.result:
  Update result file.
mysql-test/t/variables.test:
  A test case for Bug#34820: log_output can be set to illegal value.
sql/set_var.cc:
  Don't allow empty values if it is prohibitted.
sql/set_var.h:
  Add a flag (m_allow_empty_value) telling if an empty value is
  acceptable for this variable.