Improvements to our data models

Usually implemented in the <subsystem>/model.py files.

In the current situation (1.0.x/1.1.1), we have different APIs and different conventions for the different models. We should try to be more consistent.

Representation of data

Standardize missing values

For tickets, when a field is unset, we currently put the value back to the empty string. This was not always the case, as we used to reset it to None / NULL which is still what we do in some situations (retargeting tickets to no milestone after a milestone delete/close). See #7691 and #11018.

We should standardize on NULL again. When retrieving NULL values from the database, we get None in Python. If needed, we can carry on this distinct meaning yet manipulate it as a string by converting None to the special value ​empty. See e.g. what we do for ticket fields​.