Fix many to many (React)

[ctamisier]

Fix the root issue of #13182
And fix an underlying issue where the relations from a many-to-many are set from the previous save.

the value from AvInput should be set once.

This same "solution" is already applied with: AvForm model={isNew ? {} : fooEntity}

[qmonmert]

and when the idList is undefined ?

[ctamisier]

It's quite tricky..
Before this PR and before the fix on mapIdList we have this "workflow":

First [1, 2, 3] is set to the value of the AvInput (even for a create new Foo, as it gets the previous one (so I just have created a Foo with [1, 2, 3] bars before getting here)...)
Then the reset is called which set the value of the AvInput to undefined.
Then when we save, values.bars is undefined (which leads to the undefined of the idList)
It looks like the AvInput gets messed up when its value changes during component rendering.

With this PR:

We set once and only once the value of AvInput which is always false (it could have been always undefined it would have worked as well).
And with this, values.bars is equal to [] which is the same case that when we save a Foo the very first time.

[ruddell]

Tested locally, looks like a better fix to me. That list should not be undefined. It also fixes the issue with the previously saved options being displayed as selected.