hugolib: Fix simple menu config

This stopped working in Hugo 0.55:

```bash
---
menu: "main"
---
```

This was also the case for using a slice of menu entries.

This still worked:

---
menu:
  main:
    weight: 30
---

navigation/pagemenus.go

@@ -60,7 +60,7 @@ func PageMenusFromPage(p Page) (PageMenus, error) {
 	if err == nil {
 		me.Menu = mname
 		pm[mname] = &me
-		return nil, nil
+		return pm, nil
 	}
 
 	// Could be a slice of strings
@@ -71,7 +71,7 @@ func PageMenusFromPage(p Page) (PageMenus, error) {
 			me.Menu = mname
 			pm[mname] = &me
 		}
-		return nil, nil
+		return pm, nil
 	}
 
 	// Could be a structured menu entry