null_string wasn't known in libmysqld/lib_sql.cc, that caused compiler's error
i put the declaration of the null_string in mysql_priv.h and renamed it
to my_null_string to reduce the probability of name's intersections


libmysqld/lib_sql.cc:
  null_string -> my_null_string
sql/mysql_priv.h:
  my_null_string was made known
sql/sql_prepare.cc:
  null_string -> my_null_string