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.

modules/FindMagnum.cmake

@@ -422,7 +422,7 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
                 MAGNUM_${_COMPONENT}_LIBRARY_RELEASE)
 
             # Reset back
-            set(CMAKE_FIND_LIBRARY_PREFIXES ${_tmp_prefixes})
+            set(CMAKE_FIND_LIBRARY_PREFIXES "${_tmp_prefixes}")
         endif()
 
         # Library location for libraries/plugins