Fix incorrect condition in statement

It should check for the json to be empty, not the stylesheet. (Stylesheet was also empty, that's why the test would pass.)

Co-authored-by: Matthijs Kuiper <info@matthijskuiper.nl>

Author: EasterPeanut

lib/ex_css_modules/ex_css_modules.ex

@@ -60,7 +60,7 @@ defmodule ExCSSModules do
 
           raise(CompileError, description: message, file: "#{filename}.json")
 
-        empty_file?(filename) ->
+        empty_file?("#{filename}.json") ->
           message =
             """
             File is empty.