Allow average round trip time to be nil

Author: p

lib/mongo/server/description.rb

@@ -189,10 +189,7 @@ class Description
       #   call took to complete.
       #
       # @since 2.0.0
-      def initialize(address, config = {}, average_round_trip_time = 0)
-        if average_round_trip_time.nil?
-          raise ArgumentError, 'Average round trip time cannot be nil'
-        end
+      def initialize(address, config = {}, average_round_trip_time = nil)
         @address = address
         @config = config
         unless unknown?