SelectLibraryConfigurations: Fix for cached <base>_LIBRARY

The line

  set( ${basename}_LIBRARY )

removes the normal variable, but if the corresponding cached variable is
present then line

  list( APPEND ${basename}_LIBRARY optimized "${_libname}" )

uses that and fails.  Replace the original line with

  set( ${basename}_LIBRARY "" )

to set the normal variable to empty instead of unsetting it.
Merge topic 'SelectLibraryConfigurations-cached-library' into next

a2099a8 SelectLibraryConfigurations: Fix for cached <base>_LIBRARY