selectedItem property of Select$ChangeEventParameters should not be optional

[biscimus]

Describe the bug

public async onChangeDimensionType(event: sap.m.Select$ChangeEvent) {
        const selectedItem = event.getParameter("selectedItem"); 
}

TS tells me that selectedItem might be undefined, which is wrong.

Expected behavior
Should return a selected item, as defined by the UI5 API reference.

[nkappler]

This bothers me alot, too, as not only this interface is affected, but virtually every event parameter interfaces members are optional, even when there are always present in practice.