Fix merging of params when empty

[skshetry]

The recent release of python-benedict==0.24.1 started ignoring empty dictionaries
during the merge. The way we expected to work was questionable and also how
benedict itself tries to handle the references to an empty dictionary is
also questionable.

I consider this to be a proper way to handle this even if this gets fixed
on the benedict side.

To demonstrate the problem with referencing an empty dictionary:

>>> x = {}
>>> y = dict(x)
>>> y.update({"foo": "bar"})
>>> x
{}
>>> y
{'foo': 'bar'}

Benedict tries to handle in-place updates, so it's tricky for it to handle this case of empty dictionaries as the references are not preserved. And, since modify_data requires updating things in-place, we require references to be preserved. So, it seems it's our duty to try to preserve references than it being left to benedict alone.

Fixes #6374.

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[skshetry]

I'll try adding creating a patch to the upstream tomorrow, and we can proceed accordingly.

[efiop]

Thank you! 🙏