hubolib: Revert to .Type = "page" when empty

This was changed with good intentions in 0.63.0.

This behaviour was not documented, but it was of course in use.

This commit rolls back to how it behaved before:

For `Page.Type` you will get:

* `type` from front matter if set.
* `.Section`
* If none of the above returns anything, return "page"

Fixes #6805

hugolib/page__meta.go

@@ -291,12 +291,18 @@ func (p *pageMeta) Title() string {
 	return p.title
 }
 
+const defaultContentType = "page"
+
 func (p *pageMeta) Type() string {
 	if p.contentType != "" {
 		return p.contentType
 	}
 
-	return p.Section()
+	if sect := p.Section(); sect != "" {
+		return sect
+	}
+
+	return defaultContentType
 }
 
 func (p *pageMeta) Weight() int {