Make sure the memory label isn't empty.

Summary:
We don't want to allow storing memories without a label, at least for now,
because it looks ugly in the listview.

Test Plan:
- Tried to save a memory without a label and got denied.
- Saved a memory with a label successfully and saw it get added t othe list.
- Restarted the app to make sure it stayed in the list.

Reviewers: jcarter

Reviewed By: jcarter

Maniphest Tasks: T66

Differential Revision: https://phabricator.edgesysdesign.com/D72