Eo base: Make parent nullable.

It's allowed to be null, so mark it as such. This is useful
for languages that support nullable vs non-nullable types.

src/lib/eo/eo_base.eo

@@ -62,7 +62,7 @@ abstract Eo.Base ()
             [[Get the parent of an object]]
          }
          values {
-            parent: Eo.Base*; [[the new parent]]
+            parent: Eo.Base * @nullable; [[the new parent]]
          }
       }
       @property event_global_freeze_count @class {