compat: Add non-None queryset attribute

compat: Add non-None queryset attribute

Older versions of Django REST Framework's 'RelatedField' did not check
for the presence 'get_queryset' before checking for a 'queryset'
attribute. This was fixed in 3.4.0 [1] and backported to the 3.3 series
in 3.3.3 [2]. However, the fix was not backported to the last release of
3.2 series [3], which is the last release to support Django 1.6. As
such, we must set a 'queryset' attribute, even if it's set to a garbage
value.

[1] ../blob/3.4.0/rest_framework/relations.py#L78-L82
[2] ../blob/3.3.3/rest_framework/relations.py#L78-L82
[3] ../blob/3.2.5/rest_framework/relations.py#L64-L67

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes-bug: #94