correct issue with useravatar where the `size` parameter was being left ...

...completely unset if neither `width` nor `height` was set.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | na
| License       | MIT
| Doc PR        | na