Do not overwrite display name and avatar URL when not empty

Created by: unknwon

In https://github.com/sourcegraph/sourcegraph/issues/15332#issuecomment-755217428:

We always update Display name and Avatar URL with the last sign in method. That means if I sign in with GitHub and GitLab in turn, my name and avatar also changes in turn.

@quinnkeast proposed in https://github.com/sourcegraph/sourcegraph/issues/15332#issuecomment-755222277:

1. First-time code host connection – fetch display name and avatar from code host if they haven't already been set on Sourcegraph.
2. Future new code host connections or sign-ins – no action.

Side: Could be a good onboarding task.