BUG#28752 (binlog_format variable is empty when binlog is turned off):

Setting MIXED format instead of UNSPECIFIED format when server is
started without log-bin.


sql/mysqld.cc:
  Setting MIXED format instead of UNSPECIFIED format when server is
  started without log-bin.

sql/mysqld.cc

@@ -3295,7 +3295,7 @@ with --log-bin instead.");
   }
     else
   {
-      global_system_variables.binlog_format= BINLOG_FORMAT_UNSPEC;
+      global_system_variables.binlog_format= BINLOG_FORMAT_MIXED;
     }
   else
     if (opt_binlog_format_id == BINLOG_FORMAT_UNSPEC)