fix: use array for single item valueRefs in conditional if test exists

[richardliu-db]

PR Description

In this change to conditional.ts, we use object instead of array if there is a single value. But I found that test must be in an array form otherwise Vega doesn't recognize it.

This PR is to fix this issue -- always use array form if test exists, even if there is only a single item.

Array

Object

Checklist

• This PR is atomic (i.e., it fixes one issue at a time).
• The title is a concise semantic commit message (e.g. "fix: correctly handle undefined properties").
• yarn test runs successfully
• For new features:
  • Has unit tests.
  • Has documentation under site/docs/ + examples.

Tips:

• https://medium.com/@greenberg/writing-pull-requests-your-coworkers-might-enjoy-reading-9d0307e93da3 is a nice article about writing a nice PR.
• Use draft PR for work in progress PRs / when you want early feedback (https://github.blog/2019-02-14-introducing-draft-pull-requests/).

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[domoritz]

Isn't this a workaround an issue in Vega?