Warning errors for unset avatar field on user model

[PavelPancocha]

We started getting a lots of "warning" errors like:

avatar field must be an ImageField/URLField on the user model, or a callable

I found out, it is because sometimes the "avatar" is not set, but it logs an error instead of just displaying the fallback icon.

[PavelPancocha]

I did fix for it: #509