expression/expressions: add null value check.
expression/expressions: add null value check.

kyle-wang/tidb,gaocegege/tidb,c4pt0r/tidb,pingcap/tidb,simon-xia/tidb,CodeRushing/tidb,yangwenmai/tidb,qhsong/tidb,XuHuaiyu/tidb,c4pt0r/tidb,c4pt0r/tidb,liyu4/tidb,EvilMcJerkface/tidb,c4pt0r/tidb,netroby/tidb,zimulala/tidb,mccxj/tidb,Nify/tidb,liyu4/tidb,pingcap/tidb,ngaut/tidb,astaxie/tidb,yangwenmai/tidb,silenceper/tidb,tiancaiamao/tidb,silenceper/tidb,zs634134578/tidb,yuanwhy/tidb,CodeRushing/tidb,yuanwhy/tidb,myrice/tidb,JinheLin/tidb,JinheLin/tidb,andelf/tidb,kyle-wang/tidb,Nify/tidb,zimulala/tidb,pingcap/tidb,mccxj/tidb,netroby/tidb,andelf/tidb,qiuyesuifeng/tidb,Cofyc/tidb,ueizhou/tidb,qhsong/tidb,tiancaiamao/tidb,8cbx/tidb,myrice/tidb,c4pt0r/tidb,qiuyesuifeng/tidb,anywhy/tidb,c4pt0r/tidb,kyle-wang/tidb,pingcap/tidb,wusphinx/tidb,ueizhou/tidb,Cofyc/tidb,spongedu/tidb,coocood/tidb,astaxie/tidb,louishust/tidb,ming-relax/tidb,ngaut/tidb,wusphinx/tidb,anywhy/tidb,silentred/tidb,spongedu/tidb,ngaut/tidb,EvilMcJerkface/tidb,ming-relax/tidb,pingcap/tidb,zs634134578/tidb,ngaut/tidb,8cbx/tidb,simon-xia/tidb,pingcap/tidb,cosmtrek/tidb,kyle-wang/tidb,ngaut/tidb,cosmtrek/tidb,kyle-wang/tidb,coocood/tidb,gaocegege/tidb,louishust/tidb,XuHuaiyu/tidb,kyle-wang/tidb,silentred/tidb,ngaut/tidb