[Haml] Allow Haml::Util.html_safe to take nil.

doc-src/HAML_CHANGELOG.md

@@ -16,6 +16,8 @@
 
 * Make loading the gemspec not crash on read-only filesystems like Heroku's.
 
+* Don't crash when methods like `form_for` return `nil` in, for example, Rails 3 beta.
+
 ## 2.2.21
 
 [Tagged on GitHub](http://github.com/nex3/haml/commit/2.2.21).

lib/haml/util.rb

@@ -210,9 +210,10 @@ module Haml
     # With older versions of the Rails XSS-safety mechanism,
     # this destructively modifies the HTML-safety of `text`.
     #
-    # @param text [String]
-    # @return [String] `text`, marked as HTML-safe
+    # @param text [String, nil]
+    # @return [String, nil] `text`, marked as HTML-safe
     def html_safe(text)
+      return unless text
       return text.html_safe if defined?(ActiveSupport::SafeBuffer)
       text.html_safe!
     end