Fixed a nil reference error in sanitize_filename

lib/technoweenie/attachment_fu.rb

@@ -371,6 +371,7 @@ module Technoweenie # :nodoc:
         end
 
         def sanitize_filename(filename)
+          return unless filename
           returning filename.strip do |name|
             # NOTE: File.basename doesn't work right with Windows paths on Unix
             # get only the filename, not the whole path

test/basic_test.rb

@@ -42,6 +42,9 @@ class BasicTest < Test::Unit::TestCase
 
     @attachment.filename = 'f o!O-.bar'
     assert_equal 'f_o_O-.bar', @attachment.filename
+    
+    @attachment.filename = nil
+    assert_nil @attachment.filename
   end
   
   def test_should_convert_thumbnail_name