keep srcset only when src is undefined

lib/single-file/html-alt-images.js

@@ -82,14 +82,16 @@ this.altImages = this.altImages || (() => {
 	function setSrc(srcData, imgElement, pictureElement) {
 		if (srcData.src) {
 			imgElement.setAttribute("src", srcData.src);
-		} else {
-			imgElement.setAttribute("src", EMPTY_IMAGE);
-		}
-		if (srcData.srcset) {
-			imgElement.setAttribute("srcset", srcData.srcset);
-		} else {
 			imgElement.setAttribute("srcset", "");
 			imgElement.setAttribute("sizes", "");
+		} else {
+			imgElement.setAttribute("src", EMPTY_IMAGE);
+			if (srcData.srcset) {
+				imgElement.setAttribute("srcset", srcData.srcset);
+			} else {
+				imgElement.setAttribute("srcset", "");
+				imgElement.setAttribute("sizes", "");
+			}
 		}
 		if (pictureElement) {
 			pictureElement.querySelectorAll("source").forEach(sourceElement => sourceElement.remove());