Fix: Cannot update value of attribute with reserved name when it starts empty

[waltton]

Hi all, first PR here 👋 .

I observed this bug on a case that I have a field with the name data and I was able to create a record and update it without any issues if the first time I create the record with a value that is not empty.
If the record is created with empty value in the data field I cannot update it via admin.

Took me a while to understand the checks so I added a comment.

At the end, my solution was just removing the check and getattr(obj, reserved_field_name, None).
I thought about adding an or condition and check if the attribute had a value on the obj or form_data but I didn't saw the point of it. Maybe I'm missing something.

[aminalaee]

Thanks for the PR.
I think it is strange this is happening since we already added some logic to rename WTForm private fields internally with WTFORMS_ATTRS_REVERSED. I think a good step would be to look why that logic is not working as it was the purpose.

[waltton]

@aminalaee I reverted the fix so the case I'm hitting is clear on the tests I added.
Please take a look at https://github.com/aminalaee/sqladmin/pull/841/checks#step:8:105

This is the issue I'm trying to fix.

If the object being passed has a empty value, the field is not denormalized.