server is started with mysqld-auto.cnf having certain variables
         persisted with NULL values server fails to start.

Analysis: In sql_show.cc:get_one_variable_ext() for variable of type string
          NULL value will be converted to empty string. This empty string is
          further stored in mysqld-auto.cnf

Fix: In sql_show.cc:get_one_variable_ext() a new out parameter is added which
     will indicate if value is NULL or empty string. This flag will futher be
     used to store correct value (NULL or empty string) in mysqld-auto.cnf.
     This patch also fixes the static privileges needed during server startup
     to set persisted variables.