sorting on a column that has a value relation is inconsistent

[tudorbarascu]

I've just noticed that sorting with a value relation as a widget gives bad results as I have NULL values in the middle.. instead of at the beginning or at the end:

As you can see from the image above, for the strada_id field which is coming from a joined layer and the widget is a value relation sorting has NULL values in the middle.
This is very bad as this disrupts the following workflow:

• Order by column...
• Select some values... based on alphabetical order (if you see NULL.. you stop.. as you would expect thee aren't values after it..)
• Edit the features using field calculator (or some other stuff)

How to Reproduce

I'll put tomorrow data + project to reproduce as I don't have the time now :(.

If I use the simple text widget instead of the value relation one the sorting is ok.

Additional context

[tudorbarascu]

Well.. l can replicate now. The NULL for value map widgets pops in the middle when sorting as sorting is done alphabetically and N from NULL is alphabetically somewhere in the midlle. I really would have expected NULL values to be be on top or at the end when sorted. But I guess..each day you learn new things.

Here's the test project with geopackage data [bug_sorting.zip].(https://github.com/qgis/QGIS/files/4565949/bug_sorting.zip).
You can see from the image below that sorting takes NULL into account alphabetically.

Is this expected or a disturbance in the force?