Fix blank favicon if root-tiddler=$:/core/save/lazy-images is set (#5423)

If image lazy loading is used with node.js the favicon is blank. The
line `-[!is[system]is[image]]` excludes only non system images from
begin saved as full tiddlers. But the `[is[image]]` line includes system
images as skinny tiddlers. The created HTML file has all system image
tiddlers (as the favicon) listed twice. And the skinny tiddler seems to
win in this case and breaks the display of the favicon.

This patch fixes this issue by excluding system images from the skinny
tiddlers list.

core/templates/save-lazy-images.tid

@@ -4,6 +4,6 @@ title: $:/core/save/lazy-images
 [is[tiddler]] -[prefix[$:/state/popup/]] -[[$:/HistoryList]] -[[$:/boot/boot.css]] -[type[application/javascript]library[yes]] -[[$:/boot/boot.js]] -[[$:/boot/bootprefix.js]] -[!is[system]is[image]] +[sort[title]] 
 \end
 \define skinnySaveTiddlerFilter()
-[is[image]]
+[!is[system]is[image]]
 \end
 {{$:/core/templates/tiddlywiki5.html}}