lib/drb/extservm.rb (service): do not return `false'

	invoke_service_command may set entries in @servers to `false',
	making it incompatible with the intended use of the
	safe navigation operator.

	This caused occasional DRb test failures, but they were hidden
	with automatic retry.

	[ruby-core:87524] [Bug #14856]

	Fixes: r53111 ("use safe navigation operator")
		commit 059c9c1c [GH-1142]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@64128 b2dd03c8-39d4-4d8f-98ff-823fe69b080e