typst template: put title block in a conditional.

This avoids an empty block for documents that lack metadata
information.  The empty block causes problems if `#set page`
is used, as it will cause a page break.

Closes #11529.

data/templates/template.typst

@@ -73,6 +73,9 @@
     }
   }
 
+#let has-title-block = title != none or (authors != none and authors != ()) or
+                       date != none or abstract != none
+#if has-title-block {
   place(top, float: true, scope: "parent", clearance: 4mm, block(below: 1em, width: 100%)[
     #if title != none {
       align(center, block[
@@ -114,6 +117,6 @@
       ]
     }
   ])
-
+}
   doc
 }