let the default function in button_to_function be nil

I don't know which is the use case for button_to_function(name)
but there's a test for it. I am focused now on RJS extraction
and do not want to introduce a backwards incompatible change
at this moment. Perhaps worth revisiting when the whole thing
is done.

actionpack/lib/action_view/helpers/javascript_helper.rb

@@ -105,7 +105,7 @@ def javascript_cdata_section(content) #:nodoc:
       #   button_to_function "Greeting", "alert('Hello world!')", :class => "ok"
       #   # => <input class="ok" onclick="alert('Hello world!');" type="button" value="Greeting" />
       #
-      def button_to_function(name, function='', html_options={})
+      def button_to_function(name, function=nil, html_options={})
         onclick = "#{"#{html_options[:onclick]}; " if html_options[:onclick]}#{function};"
 
         tag(:input, html_options.merge(:type => 'button', :value => name, :onclick => onclick))