fix(layout): Auto package annotation matches other elements

Author: machow

quartodoc/layout.py

@@ -217,7 +217,7 @@ class Auto(_Base):
     exclude: Optional[str] = None
     dynamic: Union[bool, str] = False
     children: ChoicesChildren = ChoicesChildren.embedded
-    package: Optional[str] = MISSING()
+    package: Union[str, None, MISSING] = MISSING()
 
 
 # TODO: rename to Default or something