Don't overwrite @authCallback in provider if null
Don't overwrite @authCallback in provider if null

When we call `newInFolder`, we call `authorized` with a callback to
save the file after authorization.

Unfortunately, the menubar also calls `authorized` right after with no
callback, overwriting our previous callback.

Now we only overwrite @authCallback when not null.

concord-consortium/cloud-file-manager,concord-consortium/cloud-file-manager,concord-consortium/cloud-file-manager,concord-consortium/cloud-file-manager