FXL: don't use book viewport if it's empty

Fixes https://github.com/johnfactotum/foliate-js/issues/52#issuecomment-2956113877

fixed-layout.js

@@ -18,7 +18,7 @@ const getViewport = (doc, viewport) => {
 
     // fallback to book's viewport
     if (typeof viewport === 'string') return parseViewport(viewport)
-    if (viewport) return viewport
+    if (viewport?.width && viewport.height) return viewport
 
     // if no viewport (possibly with image directly in spine), get image size
     const img = doc.querySelector('img')