Fix server error if username is none

[alistairewj]

We're receiving a lot of TypeError due to, presumably, crawlers trying to login using token based auth (e.g. user agent Microsoft Excel 2014). This is a quick fix to prevent the server errors so we can investigate further.

Ref: #2034

[tompollard]

@bemoody pointed out that we can reproduce the error with:

curl -H user-agent:'Microsoft Office Word 2014' -H accept-auth:'badger,Wlid1.1,Bearer,Basic,NTLM,Digest,Kerberos,Negotiate,Nego2' -H authorization:'Bearer' localhost:8000

He suggested that:

1. Possibly our custom authorization (DualAuthModelBackend) is not behaving in the way that Django expects it to behave. We should look at the current authorization module and compare to our implementation.
2. It's possible that the fix in this PR will break OAuth.
3. We need to better understand the OAuth module and work out what it is doing. We could consider temporarily removing the OAuth middleware in the meantime.

[alistairewj]

2. It's possible that the fix in this PR will break OAuth.

I think it absolutely will, but no one is using OAuth right now AFAIK?

[tompollard]

I think it absolutely will, but no one is using OAuth right now AFAIK?

Right now no, but we will still need to fix this. I raised an issue at: #2040