[rubygems/rubygems] fix dangling empty hooks

it turns out that running `bundle plugin uninstall some-plugin` would remove that plugin from the list of hooks, but if the list of hooks for an event was now empty, we would serialize the empty array into yaml as an empty single bullet item. which would then get unserialized as a plugin with the name empty string. which we would then try to load and explode. 😬

https://github.com/rubygems/rubygems/commit/545ebba9a5

lib/bundler/plugin/index.rb

@@ -74,7 +74,10 @@ module Bundler
       def unregister_plugin(name)
         @commands.delete_if {|_, v| v == name }
         @sources.delete_if {|_, v| v == name }
-        @hooks.each {|_, plugin_names| plugin_names.delete(name) }
+        @hooks.each do |hook, names|
+          names.delete(name)
+          @hooks.delete(hook) if names.empty?
+        end
         @plugin_paths.delete(name)
         @load_paths.delete(name)
         save_index

spec/bundler/bundler/plugin/index_spec.rb

@@ -98,6 +98,12 @@ RSpec.describe Bundler::Plugin::Index do
       expect(index.hook_plugins("after-bar")).to eq([plugin_name])
     end
 
+    it "is gone after unregistration" do
+      expect(index.index_file.read).to include("after-bar:\n  - \"new-plugin\"\n")
+      index.unregister_plugin(plugin_name)
+      expect(index.index_file.read).to_not include("after-bar:\n  - \n")
+    end
+
     context "that are not registered" do
       let(:file) { double("index-file") }