`group-by` should accept `?` for optional path members

[rgwood]

group-by fails if the specified column is not present on any rows:

> [{'a': 1}, {'a': 2} {}] | group-by a
Error: nu::shell::column_not_found

  × Cannot find column
   ╭─[entry #6:1:1]
 1 │ [{'a': 1}, {'a': 2} {}] | group-by a
   ·                     ─┬    ────┬───
   ·                      │        ╰── cannot find column 'a'
   ·                      ╰── value originates here
   ╰────

Would be nice if group-by worked with ? so you could do group-by a? and it would ignore any rows without an a column.

Context: this came up as a usability issue on Discord https://discord.com/channels/601130461678272522/614593951969574961/1100648770326233199