Fall back to default avatar in RR when member isn't loaded yet

[bwindels]

As opposed to not rendering it at all, which makes all the RR pop in when the members are loaded, as reported in linked bug.

Fixes element-hq/element-web#7260

[bwindels]

Thanks. Getting an error in Pills I want to look into before merging this:

Warning: Failed prop type: The prop `name` is marked as required in `BaseAvatar`, but its value is `undefined`.
    in BaseAvatar (created by MemberAvatar)
    in MemberAvatar (created by Pill)
    in a (created by Pill)
    in Pill

[bwindels]

Can't repro it now, and should not throw in any case, so merging.