executor: refine the log msg when 'set character_set_results=null' (#5052)
executor: refine the log msg when 'set character_set_results=null' (#5052)


kyle-wang/tidb,kyle-wang/tidb,pingcap/tidb,spongedu/tidb,c4pt0r/tidb,c4pt0r/tidb,ngaut/tidb,ngaut/tidb,pingcap/tidb,ming-relax/tidb,ngaut/tidb,XuHuaiyu/tidb,Cofyc/tidb,Cofyc/tidb,CodeRushing/tidb,kyle-wang/tidb,qiuyesuifeng/tidb,pingcap/tidb,tiancaiamao/tidb,EvilMcJerkface/tidb,ming-relax/tidb,wusphinx/tidb,yangwenmai/tidb,EvilMcJerkface/tidb,yangwenmai/tidb,c4pt0r/tidb,coocood/tidb,kyle-wang/tidb,kyle-wang/tidb,CodeRushing/tidb,qiuyesuifeng/tidb,kyle-wang/tidb,wusphinx/tidb,spongedu/tidb,tiancaiamao/tidb,ngaut/tidb,ngaut/tidb,Nify/tidb,c4pt0r/tidb,pingcap/tidb,pingcap/tidb,c4pt0r/tidb,mccxj/tidb,mccxj/tidb,coocood/tidb,pingcap/tidb,cosmtrek/tidb,c4pt0r/tidb,Nify/tidb,cosmtrek/tidb,XuHuaiyu/tidb,ngaut/tidb