Fix that resetting the profile picture to system default does not sync to SSO client sites

[tanius]

In our installation, user profile pictures are synced from a Discourse SSO provider site to our Discourse SSO client sites upon login. This works well, but does not work in the following case:

User changes profile picture on the SSO provider Discourse site, from a custom picture to be again "system default" (a letter on a coloured background). This applies fine on the SSO provider site, but is not synced to any Discourse SSO client sites.

As a visually equivalent workaround, you can save the system default profile picture on your computer and upload that as a new custom profile picture. It will be synced to SSO client sites on the next login.

(This is nothing important, just a note to us to test this again when we upgrade to the latest version of Discourse and to report this upstream if the issue is still found there.)

[tanius]

After the recent Discourse update to 2.7.11, which is still the most recent version as of today, this issue still exists. So it's time to report it upstream – I just did, it's now tracked upstream here.

(Note that there is also a second, independent issue affecting profile pictures, where a user's profile picture will not sync at all to connected Discourse SSO client sites. Only experienced by the Discourse user of @damingo so far. This issue is not treated here, and we have not yet found a cause for it.)