Refactor RegularLink: Do not fake description if none is given

To more faithfully handle inline images we need to know whether the original
link included a description - being more explicit about that will make it
easier.

see org.el/org-display-inline-images

> An inline image is a link which follows either of these
> conventions:
>
>   1. Its path is a file with an extension matching return value
>      from `image-file-name-regexp' and it has no contents.
>
>   2. Its description consists in a single link of the previous
>      type.

org/html.go

@@ -188,9 +188,12 @@ func (w *HTMLWriter) writeFootnoteLink(l FootnoteLink) {
 
 func (w *HTMLWriter) writeRegularLink(l RegularLink) {
 	url := html.EscapeString(l.URL)
-	descriptionWriter := w.emptyClone()
-	descriptionWriter.writeNodes(l.Description...)
-	description := descriptionWriter.String()
+	description := url
+	if l.Description != nil {
+		descriptionWriter := w.emptyClone()
+		descriptionWriter.writeNodes(l.Description...)
+		description = descriptionWriter.String()
+	}
 	switch l.Protocol {
 	case "file":
 		url = url[len("file:"):]