[Form] Let null values to unset fields in PATCH requests

I've changed the way form checks wherever or not to submit childs by checking submitted data with array_key_exists and not with just isset.

This way in PATCH request values are not processed when they are not in array and not also when the value is null otherwise there is no way to null a value with a PATCH request, even passing it. Null check is still done using isset on next line passing null if the value is not set.

This can lead to some BC breaks depending on how users used form in their code.