img: only provide alt text if it was specified

if suitable alternate text is unknown, then it should not be given.
empty alt text is suitable mainly for purely decorative images.
(cherry picked from commit 3cd7f67f0cf894f4fd5ba16f68e82e4f7bdbfdc5)

IkiWiki/Plugin/img.pm

@@ -119,9 +119,9 @@ sub preprocess (@) {
 	}
 
 	my $imgtag='<img src="'.$imgurl.
-		'" alt="'.(exists $params{alt} ? $params{alt} : '').
 		'" width="'.$im->Get("width").
 		'" height="'.$im->Get("height").'"'.
+		(exists $params{alt} ? '" alt="'.$params{alt}.'"' : '').
 		(exists $params{title} ? ' title="'.$params{title}.'"' : '').
 		(exists $params{class} ? ' class="'.$params{class}.'"' : '').
 		(exists $params{id} ? ' id="'.$params{id}.'"' : '').