[INTERNAL][FIX] sap.ui.fl : Correct getChangesForVariant
[INTERNAL][FIX] sap.ui.fl : Correct getChangesForVariant

In case of no change is available for a variant, the correct entry in
VariantChanges map should be returned instead of empty object.

BCP: 1870108265
Change-Id: Ia26759ff4f509b68f1d567d2731071b31545ccef

SAP/openui5,SAP/openui5,cschuff/openui5,cschuff/openui5,SAP/openui5,SAP/openui5,cschuff/openui5