Allow traversing nullable related fields

[gcbirzan]

This allows you to have a related field that traverses nullable relations. A default is required to have the value in the output, but the current behaviour is to throw an AttributeError.

Closes #5848

[carltongibson]

Hi @gcbirzan. This looks very much in line with the other changes we've made thus far here, following from #5375.

I just want to look at the test case a little more closely. Specifically, how it differs from the test case in #5880 and whether we can combine the two.

[carltongibson]

Hi @gcbirzan.

The difference in the tests was just exactly that you're one was using RelatedField whereas the previous one had used an CharField.

I adjusted the test to use the existing models. This looks good to me. Thanks for the input!