Fix/undefined index

[germain-italic]

It is recommended to merge #73 before before this one.

---

This PR will fix many "Undefined index" errors that appear on PHP 8.1 with error_reporting set to ALL (which is usually the case on localhost).

[germain-italic]

Also in this commit I've added 2 example schemas:

• One with all possible field types
• One with a table without primary key

They can be imported in 1 click (each) thanks to the #70 feature/import-schema.

I've noticed that some field types are not correctly supported so I've created issue #75 (not sure when/if I'll work on it thought).

Tables without PK are partially supported, see issue #79.