LTI 1.3 $Canvas.assignment.dueAt.iso8601 is null rather than unexpanded

[drlippman]

Summary:

Past behavior, and the expected behavior per documentation, is that if an LTI 1.3 external tool assignment has no due date, then the $Canvas.assignment.dueAt.iso8601 substitution variable is unexpanded. Recently (in the last few days), Canvas started sending null instead of the unexpanded string.

Steps to reproduce:

1. Set up an LTI 1.3 tool with a custom field using the substitution variable $Canvas.assignment.dueAt.iso8601, like canvas_assignment_due_at=$Canvas.assignment.dueAt.iso8601
2. Create an external tool assignment using that tool, and set no due date on it
3. Launch the tool, capture and decode the id_token containing the launch claims
4. Observe the variable value is null instead of a string

Expected behavior:

That the claim include

"https://purl.imsglobal.org/spec/lti/claim/custom": {
    "canvas_assignment_due_at": "$Canvas.assignment.dueAt.iso8601"
}

Actual behavior:

The claim includes

"https://purl.imsglobal.org/spec/lti/claim/custom": {
    "canvas_assignment_due_at": null
}

Additional notes:

I'm assuming this is related to the recent commit 5a74bc9, but I'm not certain of that.