filter resources with empty URLs

lib/single-file/single-file-core.js

@@ -833,6 +833,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 	const DATA_URI_PREFIX = "data:";
 	const BLOB_URI_PREFIX = "blob:";
 	const ABOUT_BLANK_URI = "about:blank";
+	const NOT_EMPTY_URL = /https?:\/\/.+/;
 	const REGEXP_URL_FN = /(url\s*\(\s*'(.*?)'\s*\))|(url\s*\(\s*"(.*?)"\s*\))|(url\s*\(\s*(.*?)\s*\))/gi;
 	const REGEXP_URL_SIMPLE_QUOTES_FN = /^url\s*\(\s*'(.*?)'\s*\)$/i;
 	const REGEXP_URL_DOUBLE_QUOTES_FN = /^url\s*\(\s*"(.*?)"\s*\)$/i;
@@ -870,7 +871,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 		}
 
 		static testValidPath(resourceURL) {
-			return !resourceURL.startsWith(DATA_URI_PREFIX) && !resourceURL.startsWith(BLOB_URI_PREFIX) && resourceURL != ABOUT_BLANK_URI;
+			return !resourceURL.startsWith(DATA_URI_PREFIX) && !resourceURL.startsWith(BLOB_URI_PREFIX) && resourceURL != ABOUT_BLANK_URI && resourceURL.match(NOT_EMPTY_URL);
 		}
 
 		static matchImport(stylesheetContent) {