Use null instead of undefined to represent "not yet ready" data in the usecases selector.

[garronej]

I should do this otherwise this kind of issues can arise if I'm not focussed enought:

#804

Using null instead like what I did in this usecase solves it:

https://github.com/InseeFrLab/onyxia/blob/main/web/src/core/usecases/s3ConfigCreation/selectors.ts

This way we don't have to think to much about what entries must be listed here (to know if the value when ready can also be undefined):

onyxia/web/src/core/usecases/s3ConfigCreation/selectors.ts

Lines 282 to 287 in b910581

	assert(formValues !== null);
	assert(connectionTestStatus !== null);
	assert(formValuesErrors !== null);
	assert(isFormSubmittable !== null);
	assert(urlStylesExamples !== null);
	assert(isEditionOfAnExistingConfig !== null);