use href attribute for<use> if xlink:href is undefined

Former-commit-id: 14d17956bae008dda7e7471757114f21de8249c8

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

@@ -1768,9 +1768,13 @@ this.singlefile.lib.core = this.singlefile.lib.core || (() => {
 		}
 
 		static async processXLinks(resourceElements, baseURI, options, batchRequest) {
-			const attributeName = "xlink:href";
+			let attributeName = "xlink:href";
 			await Promise.all(Array.from(resourceElements).map(async resourceElement => {
-				const originalResourceURL = resourceElement.getAttribute(attributeName);
+				let originalResourceURL = resourceElement.getAttribute(attributeName);
+				if (originalResourceURL == null) {
+					attributeName = "href";
+					originalResourceURL = resourceElement.getAttribute(attributeName);
+				}
 				let resourceURL = Util.normalizeURL(originalResourceURL);
 				if (Util.testValidPath(resourceURL) && !Util.testIgnoredPath(resourceURL)) {
 					resourceElement.setAttribute(attributeName, EMPTY_IMAGE);