contrib/wix/defines.wxi

templatekw: clarify the result of {latesttag} when no tag exists

My initial expectation was that the list would be empty, and therefore
detectable with {if()}.  The map for {latesttag()} is populated with real values
in this case (except {tag}), so it probably doesn't make any sense to change
this to an empty list.

<Include>

  <?if $(var.Platform) = "x64" ?>
    <?define IsX64 = yes ?>
  <?else?>
    <?define IsX64 = no ?>
  <?endif?>

</Include>