Fix null values cause warning in forms

[fzaninotto]

Closes #7782

[slax57]

This doesn't work for nested values, like author.name for instance

[fzaninotto]

So the fix requires a bit of recursion:

export default function getFormInitialValues(
    defaultValues: DefaultValue,
    record?: Partial<RaRecord>
) {
    const finalInitialValues = merge(
        {},
        getValues(defaultValues, record),
        record
    );
    // replace null values by empty string to avoid controlled/ uncontrolled input warning
    return mapValuesDeep(finalInitialValues, value =>
        value === null ? '' : value
    );
}

const mapValuesDeep = (obj, callback) =>
    mapValues(obj, v =>
        isPlainObject(v) ? mapValuesDeep(v, callback) : callback(v)
    );

But I'm afraid it could have nasty side effects. Why should we modify the inner properties of embedded objects - some of which my not be modifiable? Besides, the sanitizeEmptyValues doesn't work for nested properties, so the empty string would be sent back to the server.

All in all, this is another corner case, which already exists today, and I don't think this PR makes it worse.

[WiXSL]

Apart from @slax57 review seems good!