`M.callWhen().optional(...)` causes rejections

[michaelfig]

Describe the bug

Steps to reproduce

Many Agoric SDK unit tests are failing with the same kind of error.

not ok 36 - name-hub › nameAdmin provideChild %ava-dur=40ms
  ---
    name: AssertionError
    message: Rejected promise returned by test
    values:
      'Rejected promise returned by test. Reason:': |-
        TypeError {
          message: 'Cannot read properties of undefined (reading \'length\')',
        }
    at: 'async packages/vats/test/test-name-hub.js:185:17'
  ...

Investigating packages/vats/test/test-name-hub.js:185:17 leads eventually to the following InterfaceGuard.

Furthermore, this error could be reproduced by calling a method guarded by M.callWhen().optional(M.arrayOf(M.any())).returns(), namely a callWhen with an optional arrayOf.

Expected behavior

No error.

Platform environment

Agoric SDK CI

Additional context

Screenshots