don't set media attribute if empty

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

@@ -831,7 +831,9 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 				const options = { maxResourceSize: this.options.maxResourceSize, maxResourceSizeEnabled: this.options.maxResourceSizeEnabled, charSet: this.charSet };
 				const stylesheetContent = await DomProcessorHelper.resolveLinkStylesheetURLs(linkElement.href, this.baseURI, options);
 				const styleElement = this.doc.createElement("style");
-				styleElement.media = linkElement.media;
+				if (linkElement.media) {
+					styleElement.media = linkElement.media;
+				}
 				styleElement.textContent = stylesheetContent;
 				linkElement.parentElement.replaceChild(styleElement, linkElement);
 			}));