Treat input as :string if attached column.type is empty

`#column_for` can return `ActiveRecord::ConnectionAdapters::NullColumn`. `column.type` will be `nil` then.
This fix makes the InputHelper#default_input_type recognize such column as string by default, just as the whole `column` would be `nil`.

Fixes the `Unable to find input class Input` error (formtastic#1183).

lib/formtastic/helpers/input_helper.rb

@@ -261,7 +261,8 @@ def default_input_type(method, options = {}) # @private
           return :file    if is_file?(method, options)
         end
 
-        if column = column_for(method)
+        column = column_for(method)
+        if column && column.type
           # Special cases where the column type doesn't map to an input method.
           case column.type
           when :string