otter assign skips Gradescope username/password request even when token is set to None or null

[champoll]

Describe the bug
When the notebook metadata contains the line token: '' , otter assign does not ask for the Gradescope username/password. The same is true if the line token: None or token: null.

To Reproduce
Add the line above to the metadata of a notebook and run "otter assign" on the notebook.

Expected behavior
Since Gradescope tokens are nonempty strings, in all these cases otter assign should assume that a Gradescope token has not been specified, and should ask for Gradescope username/password.

Versions
Python version: 3.8.5
Otter-Grader version: 3.0.6

Additional context
A workaround is to comment out the line. However, I am trying to create a file that contains all of the metadata settings and it would be convenient to be able to include the "token" line.

[chrispyles]

Implemented a fix in #374 which will be included in v3.1.0.