Handle null in Activity#requestPermissions optimization

The recent optimization introduced in ag/30469303
caused NPE. App may pass null as an element accidentally
in `permissions` collection parameter, the optimization
shouldn't change the behavior.

Fix: 383954592
Test: atest android.permissionui.cts.PermissionTest23#testNullAndRealPermission
FLAG: EXEMPT bug fix
Change-Id: Ic271664391afaa3512c1e34f26ef7e792e03213f