fixed detection of empty contents

lib/single-file/modules/css-fonts-alt-minifier.js

@@ -36,7 +36,7 @@ this.singlefile.lib.modules.fontsAltMinifier = this.singlefile.lib.modules.fonts
 	const REGEXP_FONT_FORMAT = /\.([^.?#]+)((\?|#).*?)?$/;
 	const REGEXP_FONT_FORMAT_VALUE = /format\((.*?)\)\s*,?$/;
 	const REGEXP_FONT_SRC = /(.*?)\s*,?$/;
-	const EMPTY_URL_SOURCE = /^url\(["']?data:[^,]*,["']?\)/;
+	const EMPTY_URL_SOURCE = /^url\(["']?data:[^,]*,?["']?\)/;
 	const LOCAL_SOURCE = "local(";
 	const MEDIA_ALL = "all";
 	const FONT_WEIGHTS = {