Fix `pod repo push --help`

Fixes #5211

lib/cocoapods/command/repo/push.rb

@@ -38,7 +38,7 @@ module Pod
           @allow_warnings = argv.flag?('allow-warnings')
           @local_only = argv.flag?('local-only')
           @repo = argv.shift_argument
-          @source = config.sources_manager.sources([@repo]).first
+          @source = config.sources_manager.sources([@repo]).first unless @repo.nil?
           @source_urls = argv.option('sources', config.sources_manager.all.map(&:url).join(',')).split(',')
           @podspec = argv.shift_argument
           @use_frameworks = !argv.flag?('use-libraries')

spec/unit/command_spec.rb

@@ -12,6 +12,7 @@ module Pod
       Command.parse(%w(repo list       )).should.be.instance_of Command::Repo::List
       Command.parse(%w(repo update     )).should.be.instance_of Command::Repo::Update
       Command.parse(%w(repo remove     )).should.be.instance_of Command::Repo::Remove
+      Command.parse(%w(repo push --help)).should.be.instance_of Command::Repo::Push
       Command.parse(%w(setup           )).should.be.instance_of Command::Setup
       Command.parse(%w(spec create     )).should.be.instance_of Command::Spec::Create
       Command.parse(%w(spec lint       )).should.be.instance_of Command::Spec::Lint