Preserve missing dependencies when node is updated

Since we moved the list of missingDependencies to fluxObject in ef8b0c47, this list has to be set each time we store a new object in the tree, otherwise it will be empty if the list of object dependencies has not changed.

For that purpose, we copy missingDepencies from previous FluxObject in that case.

This is required in the context of sylva-projects/sylva-core!3326 (merged)