fix(deposit): page render and save a empty record

[philippgualdi]

Problem

The metadata structure for deposit has changed, but the backend is still sending the previous format, which lacks the fields key. This causes serialization errors and results in empty records being saved, as the backend attempts to process incomplete metadata.

Solution

This fix updates the serializer to handle cases where the fields key is missing from the metadata. By marking fields as optional, the serializer can now handle responses from the backend that omit this key, preventing the creation and saving of empty records.