Fix broken rmdir script

Unfortunately, the removal shell script introduced in #10860 does not handle paths very well that dont exist, e.g.

* `find` runs before its `-exec` test, thus throws `find: "${path}": No such file or directory`
* it seem that `/bin/rmdir` is intended to break is certain cases, thus `-f` is not desired.
  so, if `${path}` does not exist, it'll still break, which is most likely not one of those cases.

This change reintroduces a check for existence. This way, it is ensured that there is actually
a directory to be removed when invoking the script.

Library/Homebrew/cask/pkg.rb

@@ -135,6 +135,8 @@ module Cask
 
     sig { params(path: T.any(Pathname, T::Array[Pathname])).void }
     def rmdir(path)
+      return unless path.exist?
+
       @command.run!(
         "/usr/bin/xargs",
         args:  ["-0", "--", "/bin/bash", "-c", RMDIR_SH, "--"],