Only update ._name_and_description if .description is set at some point
Only update ._name_and_description if .description is set at some point

Fixes a run description deprecation warning if you call `wandb.init(id='asdf')`.

Also lets us instrument usage of the deprecated description field in the
backend. Previously we'd always set the description field when upserting a run.
Now we set it to null unless the user has explicitly set the old-style
description somehow, either through a `wandb.init()` parameter or an
environment variable.

wandb/client,wandb/client,wandb/client