fix: error by parent not menu attached but with children that is while disabled cascade menu attached

[skyclb]

Ticket

https://github.com/VirtusLab/strapi-plugin-navigation/issues/285

Summary

What does this PR do/solve?

Fixes an error occuring by parent item which is menu attached and has children. It only happens when the plugin option "Cascade menu attached" is enabled.

Test Plan

How are you testing the work you're submitting?

I created a multi-level navigation for testing purposes. I compared the default, tree and rfr renders with before and after the change from this request with each other. Everything seems to work and only the bug is fixed from my end.

Please consider making an additional test on your own, since it lays on such a crucial place where a lot comes together. I did not reverse engineer all the code in the file, only bug related areas. I can't really know what side effects could occure.