"branch -o json" fails on an empty repository

[pfw]

Describe the bug
branch -o json fails on an empty repository

To Reproduce

kart init
kart branch -o json

Expected behaviour
Not a traceback, text format works

kart branch

Output

Traceback (most recent call last):
  File "kart_cli.py", line 4, in <module>
  File "kart/cli.py", line 351, in entrypoint
  File "click/core.py", line 829, in __call__
  File "click/core.py", line 782, in main
  File "kart/cli.py", line 159, in invoke
  File "click/core.py", line 1259, in invoke
  File "click/core.py", line 1066, in invoke
  File "click/core.py", line 610, in invoke
  File "click/decorators.py", line 21, in new_func
  File "kart/branch.py", line 32, in branch
  File "kart/branch.py", line 50, in list_branches_json
_pygit2.GitError: reference 'refs/heads/main' not found
[481] Failed to execute script kart_cli

Version Info

• OS: Mac OS
• Version: v0.11.1