fix: Ungroup pathmarks in offset channels also for aggregated fields

[joelostblom]

PR Description

I missed testing with aggregated field properly in #9819, so an aggregation in the offset channel currently connects pathmarks incorrectly:

After this PR, pathmarks are correctly ungrouped also for aggregated fields:

Spec for testing:

{
  "data": {
    "values": [
      {"a": "A", "b": 28, "c": "x", "d": "m"},
      {"a": "B", "b": 55, "c": "x", "d": "m"},
      {"a": "C", "b": 43, "c": "x", "d": "n"},
      {"a": "D", "b": 91, "c": "x", "d": "n"},
      {"a": "A", "b": 88, "c": "y", "d": "m"},
      {"a": "B", "b": 55, "c": "y", "d": "m"},
      {"a": "C", "b": 43, "c": "y", "d": "n"},
      {"a": "D", "b": 55, "c": "y", "d": "n"},

    ]
  },
  "mark": {"type": "line"},
  "encoding": {
    "x": {"field": "a"},
    "yOffset": {"field": "b", "aggregate": "sum", "type": "quantitative"},
    "y": {"field": "c"}
  }
}

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").
• npm 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/).

[domoritz]

See #9819 (comment)