Item version saving pollutes item_version with lots of null values

[be-smith]

This bug has been hard to replicate.

When a block is made then a manual save triggered the block data can show up in the diff:

Steps to reproduce:

1. Make a new sample
2. Change something trivial and save by clicking the floppy disk
3. Add a block (preferably CycleBlock)
4. Attach a file
5. Click Save Item again
6. View Version History

The fix is in items.py on line 1798 for save_item to change:

item = ITEM_MODELS[item_type](**item).dict()

to

item = ITEM_MODELS[item_type](**item).dict(exclude_none=True)

But worth checking follow on effects of this change:

"Fixed" behaviour:

I believe this behaviour is already fixed in pydantic v2 due to exclude_unset=True being used there