Dump an model excluding `none` values

# Feature request

### Context

I don't want to waste database space writing empty data.

### Solution

Adding `exclude_none` paramater to `model_dump_doc` would solve the problem.

Start by locating the model_dump_doc entry point and inspect how its current parameters control document serialization. Add support for the requested exclude_none option, then verify that empty model values are omitted from the dumped document.

Written by the indexing model from the issue text.