FeatureConstructor: Don't replace missing strings with ?

[janezd]

Issue

Fixes #5982.

String values were mapped through StringVariable.str_val, which was unnecessary (is it?) and replaced missing values with "?".

Description of changes

This PR just removes mapping, so the example in #5982 gives the desired input.

This treats missing strings as something that can be worked with, e.g. added. Given that missing strings are represented by empty strings, we do not have a good way of preventing it; replacing them with None would be ... artificial.

Includes

• [X] Code changes
• [X] Tests