Return nil as attachment.path when no file assigned; don't explode.

lib/paperclip/attachment.rb

@@ -106,7 +106,7 @@ module Paperclip
     # disk. If the file is stored in S3, the path is the "key" part of the URL,
     # and the :bucket option refers to the S3 bucket.
     def path style = nil #:nodoc:
-      interpolate(@path, style)
+      original_filename.nil? ? nil : interpolate(@path, style)
     end
 
     # Alias to +url+

test/attachment_test.rb

@@ -229,6 +229,12 @@ class AttachmentTest < Test::Unit::TestCase
       assert_equal "/avatars/blah/missing.png", @attachment.url(:blah)
     end
     
+    should "return nil as path when no file assigned" do
+      assert @attachment.to_file.nil?
+      assert_equal nil, @attachment.path
+      assert_equal nil, @attachment.path(:blah)
+    end
+    
     context "with a file assigned in the database" do
       setup do
         @attachment.stubs(:instance_read).with(:file_name).returns("5k.png")