remove source tag regardless the presence of the src attribute

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

@@ -472,7 +472,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 					}
 				});
 			}
-			const videoSourceElements = this.doc.querySelectorAll("video[src], video > source[src]");
+			const videoSourceElements = this.doc.querySelectorAll("video[src], video > source");
 			this.stats.set("processed", "video sources", videoSourceElements.length);
 			if (this.options.removeVideoSrc) {
 				this.stats.set("discarded", "video sources", videoSourceElements.length);