refresh_from_db is breaking on null foreign keys

[Johannes Lerch]

If i try to call refresh_from_db on a model instance that has a foreignkey set to null, i get an error:

(stacktrace was removed after ticket was resolved)

here rel_instance is assigned:
​https://github.com/django/django/blob/stable/1.8.x/django/db/models/base.py#L630

if it is None (foreignkey not set, as it is nullable in this model) the getattr function will complain that it can not get 'id' from 'NoneType'

[Tim Graham]

Reproduced with the attached test.

[Claude Paroz <claude@…>]

In 5cf96b49e43daea6d4a0ba1c80c45e90c74f4e47:

Fixed #24418 -- Prevented crash in refresh_from_db with null fk

Thanks Johannes Lerch for the report, Tim Graham for the test case,
and Simon Charette for the review.

[Claude Paroz <claude@…>]

In b825ec38c1e16a271c69694562963eadbe0c6352:

[1.8.x] Fixed #24418 -- Prevented crash in refresh_from_db with null fk

Thanks Johannes Lerch for the report, Tim Graham for the test case,
and Simon Charette for the review.
Backport of 5cf96b49e4 from master.