Do not turn on keyword_init for Struct subclass if keyword hash is
	 empty

	This was accidentally turned on because there was no checking for
	Qundef.

	Also, since only a single keyword is currently supported, simplify
	the rb_get_kwargs call.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67841 b2dd03c8-39d4-4d8f-98ff-823fe69b080e