update-third-party: remove empty directories as well

The `git ls-files | xargs rm` removes only files; directories which are
empty are left laying around. This later chokes the `mv` which puts the
"reduced" directory into place. Remove the empty directories as well.

Utilities/Scripts/update-third-party.bash

@@ -85,6 +85,7 @@ if [ -n "$basehash" ]; then
     # Clear out the working tree
     pushd "$extractdir"
     git ls-files | xargs rm -v
+    find . -type d -empty -delete
     popd
 else
     # Create a repo to hold this package's history