Make edges and nodes on JoblistingConnection nonNullable

[LudvigHz]

With DjangoFilterConnectionField, neither edges or nodes will ever be null. These being nullable gives extra work when using the schema, since we will have to unnecessarily check for null a lot.

Currently, our schema looks like this:

joblistings(
  orderBy: [OrderByJoblistingType]
  ...
): JoblistingConnection

# JoblistingConnection:
...
edges: [JoblistingEdge]!
...

# JoblistingEdge:

node: Joblisting

What we want:

joblistings(
  orderBy: [OrderByJoblistingType]
  ...
): JoblistingConnection

# JoblistingConnection:
...
edges: [JoblistingEdge!]!
...

# JoblistingEdge:

node: Joblisting!

It might be an idea to make the JoblistingConnection itselt nonnullable as well, since this is only null in the event that we also get an error. (i.e. illegal arguments).

See also: graphql-python/graphene-django#901