refactored font validation test

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

@@ -1311,12 +1311,11 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 							if (!Util.testIgnoredPath(resourceURL)) {
 								if (Util.testValidURL(resourceURL, baseURI, options.url)) {
 									let { content } = await batchRequest.addURL(resourceURL, true);
-									let invalidResource = content == EMPTY_DATA_URI || content.startsWith(PREFIX_DATA_URI_VND) || content.startsWith(PREFIX_DATA_URI_IMAGE_SVG);
-									if (!invalidResource) {
-										invalidResource = !(await DocUtil.validFont(urlFunction));
-									}
-									if (invalidResource) {
-										content = EMPTY_DATA_URI;
+									if (content != EMPTY_DATA_URI && !content.startsWith(PREFIX_DATA_URI_VND) && !content.startsWith(PREFIX_DATA_URI_IMAGE_SVG)) {
+										const validResource = await DocUtil.validFont(urlFunction);
+										if (!validResource) {
+											content = EMPTY_DATA_URI;
+										}
 									}
 									declaration.value.children.forEach(token => {
 										if (token.type == "Url" && DocUtil.removeQuotes(Util.getCSSValue(token.value)) == originalResourceURL) {