Kart breaks when geojson output is used in conflicts command and conflict only affects schemas

[volaya]

To reproduce:

• Modify a dataset changing its schema
• Make a different schema change in a different branch
• Merge branches.

The merge operation will result in conflicts.

Run kart conflicts -ogeojson

The following error is raised:

Traceback (most recent call last):
  File "kart_cli.py", line 4, in <module>
  File "kart\cli.py", line 340, in entrypoint
  File "lib\site-packages\click\core.py", line 829, in __call__
  File "lib\site-packages\click\core.py", line 782, in main
  File "kart\cli.py", line 163, in invoke
  File "lib\site-packages\click\core.py", line 1259, in invoke
  File "lib\site-packages\click\core.py", line 1066, in invoke
  File "lib\site-packages\click\core.py", line 610, in invoke
  File "lib\site-packages\click\decorators.py", line 21, in new_func
  File "kart\conflicts.py", line 294, in conflicts
  File "kart\conflicts.py", line 86, in list_conflicts
  File "kart\conflicts.py", line 214, in conflicts_json_as_geojson
TypeError: list indices must be integers or slices, not str
[18628] Failed to execute script kart_cli

It doesn'tmake sense to use the geojson output format in this case, but I guess that the command should fail more gracefully.