MDEV-20744 SET GLOBAL `replicate_do_db` = DEFAULT causes crash.

DEFAULT for the replicate_do_db is the "" as our documentation states.

mysql-test/suite/sys_vars/r/replicate_do_db_basic.result

@@ -37,5 +37,9 @@ SET @@GLOBAL.replicate_do_db=null;
 SELECT @@GLOBAL.replicate_do_db;
 @@GLOBAL.replicate_do_db
 
+SET @@GLOBAL.replicate_do_db=DEFAULT;
+SELECT @@GLOBAL.replicate_do_db;
+@@GLOBAL.replicate_do_db
+
 # Cleanup.
 SET @@GLOBAL.replicate_do_db = @save_replicate_do_db;

mysql-test/suite/sys_vars/t/replicate_do_db_basic.test

@@ -38,5 +38,8 @@ SELECT @@GLOBAL.replicate_do_db;
 SET @@GLOBAL.replicate_do_db=null;
 SELECT @@GLOBAL.replicate_do_db;
 
+SET @@GLOBAL.replicate_do_db=DEFAULT;
+SELECT @@GLOBAL.replicate_do_db;
+
 --echo # Cleanup.
 SET @@GLOBAL.replicate_do_db = @save_replicate_do_db;

sql/sys_vars.ic

@@ -614,7 +614,11 @@ public:
   { DBUG_ASSERT(FALSE); }
 
   void global_save_default(THD *thd, set_var *var)
-  { DBUG_ASSERT(FALSE); }
+  {
+    char *ptr= (char*)(intptr)option.def_value;
+    var->save_result.string_value.str= ptr;
+    var->save_result.string_value.length= ptr ? strlen(ptr) : 0;
+  }
 
   bool session_update(THD *thd, set_var *var)
   {