replace invalid URls with empty image data URI

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

@@ -1189,6 +1189,8 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 						if (DomUtil.testValidURL(resolvedURL, baseURI, options.url) && resourceURL != resolvedURL && stylesheetContent.includes(urlFunction)) {
 							stylesheetContent = stylesheetContent.replace(DomUtil.getRegExp(urlFunction), urlFunction.replace(originalResourceURL, resolvedURL));
 						}
+					} else {
+						stylesheetContent = stylesheetContent.replace(DomUtil.getRegExp(urlFunction), urlFunction.replace(originalResourceURL, EMPTY_DATA_URI));
 					}
 				} else {
 					if (resourceURL.startsWith(DATA_URI_PREFIX)) {