[rubygems/rubygems] Fix unnecessary $LOAD_PATH modification in standalone script

Due to the `bundler` gem itself being ignored from `$LOAD_PATHS`, a
`nil` value is being introduced here, resulting in the current folder
being added to the `$LOAD_PATH` by the standalone script.

I'm pretty sure this is unintentional.

https://github.com/rubygems/rubygems/commit/df54b07b5e

lib/bundler/installer/standalone.rb

@@ -31,7 +31,7 @@ module Bundler
           gem_path(path, spec).sub(version_dir, '#{ruby_engine}/#{ruby_version}')
           # This is a static string intentionally. It's interpolated at a later time.
         end
-      end.flatten
+      end.flatten.compact
     end
 
     def version_dir