Modify getCursorPosition to return null for display w/o cursor

For cursor associated with a display, that is outside the
DisplayTopology, getCursorPosition will also be returned correctly.

Bug: 409180058
Test: atest inputflinger_tests
Flag: EXEMPT test API update
Change-Id: Ic2b8909a05e55fef91a6052cacc17d70b1720b5b