[BUG] GroupBy schema shapes are incorrect when using list agg on list column

[karlhigley]

Describe the bug
The shapes recorded by the GroupBy operator are incorrect when running the list aggregation on a column that already contains list values.

Steps/Code to reproduce bug
Details here: #1760 (comment)

Expected behavior
The shape should be adjusted to add the additional dimension that's created by this aggregation.

Additional context
This bug was introduced by #1760, but should be very low impact until we start using shape information directly (instead of relying on it indirectly through is_list and is_ragged.)