Looks like a bug to me. Both functions don't return NULL under any circumstances so they satisfy requrements for BINARY indexes.
>
>ONERROR
>CREATECURSOR testBinary (aa i NULL)
>* they don't have to fail because they cannot returnnull
>INDEXONEMPTY(aa) TAGbitmap BINARY
>INDEXONISNULL(aa) TAGbitmap BINARY
>