fix(entity): do not skip empty arrays

BREAKING CHANGE: toObject and toJSON will now return empty arrays instead of skipping completely

src/entity.js

@@ -640,10 +640,7 @@ function asObject(entity, shallow) {
       }
     });
 
-    // We don't send along empty arrays.
-    if (asObjects.length > 0) {
-      pojo[propertyName] = asObjects;
-    }
+    pojo[propertyName] = asObjects;
   });
 
   return pojo;