[FIX] sap.m.ComboBoxBase: use native placeholder
[FIX] sap.m.ComboBoxBase: use native placeholder

Change-Id: Ife7807bd93e846ebaab20d04e6e8fa93b45a9c6f
BCP: 1570749082

cschuff/openui5,nzamani/openui5,SQCLabs/openui5,olirogers/openui5,cschuff/openui5,SAP/openui5,SAP/openui5,nzamani/openui5,SQCLabs/openui5,SQCLabs/openui5,nzamani/openui5,olirogers/openui5,SAP/openui5,SAP/openui5,olirogers/openui5,cschuff/openui5