fixed replacement of empty url functions in stylsheets

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

@@ -1197,7 +1197,13 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 							stylesheetContent = stylesheetContent.replace(DomUtil.getRegExp(urlFunction), urlFunction.replace(originalResourceURL, resolvedURL));
 						}
 					} else {
-						stylesheetContent = stylesheetContent.replace(DomUtil.getRegExp(urlFunction), urlFunction.replace(originalResourceURL, EMPTY_DATA_URI));
+						let newUrlFunction;
+						if (originalResourceURL) {
+							newUrlFunction = urlFunction.replace(originalResourceURL, EMPTY_DATA_URI);
+						} else {
+							newUrlFunction = "url(" + EMPTY_DATA_URI + ")";
+						}
+						stylesheetContent = stylesheetContent.replace(DomUtil.getRegExp(urlFunction), newUrlFunction);
 					}
 				} else {
 					if (resourceURL.startsWith(DATA_URI_PREFIX)) {