Default to an empty collection if falsey given

This will ensure we attempt to render an empty collection, meaning we
don't actually render anything at all. Allowing `nil` or a falsey value
through results in calling `render_partial` rather than
`render_collection`, which isn't what we want.

actionview/lib/action_view/renderer/partial_renderer.rb

@@ -403,7 +403,7 @@ def setup(context, options, block)
 
     def collection_from_options
       if @options.key?(:collection)
-        collection = @options[:collection]
+        collection = @options[:collection] || []
         collection = collection.to_ary if collection.respond_to?(:to_ary)
         collection
       end