The `rcc_PREFIX` will be set to `/` if it is not passed to the
function and it is not defined in `QT_RESOURCE_PREFIX`.
I also removed an unnecessary check of the `rcc_PREFIX`.
[ChangeLog][QtCore][CMake] The `PREFIX` parameter of the
`qt_add_resources` is now optional. If not passed, and
`QT_RESOURCE_PREFIX` is not defined, `/` will be used as the path
prefix.
Fixes: QTBUG-104938
Change-Id: I6524ab5dc54f035272e4c2e3154eb67591efb650
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
