modules: properly handle platforms with empty CMAKE_FIND_LIBRARY_PREFIXES.
modules: properly handle platforms with empty CMAKE_FIND_LIBRARY_PREFIXES.

For example Windows with MSVC. The variable is set to empty list, which
we need to preserve when setting it back. Otherwise CMake complains
loudly.

MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum