Ported names fix for Ruby to the release branch.

ruby/lib/google/protobuf.rb

@@ -56,6 +56,7 @@ else
       module Internal
         def self.infer_package(names)
           # Package is longest common prefix ending in '.', if any.
+          if not names.empty?
             min, max = names.minmax
             last_common_dot = nil
             min.size.times { |i|
@@ -67,6 +68,9 @@ else
             end
           end
 
+          nil
+        end
+
         class NestingBuilder
           def initialize(msg_names, enum_names)
             @to_pos = {nil=>nil}