Do not assume channels are always supplied

[jaimergp]

Description

This command

$ conda create -n unused --dry-run --override-channels -c local python

will make conda-libmamba-solver crash with:

    Traceback (most recent call last):
      File "/opt/conda-src/conda/exception_handler.py", line 17, in __call__
        return func(*args, **kwargs)
      File "/opt/conda-src/conda/cli/main.py", line 64, in main_subshell
        exit_code = do_call(args, parser)
      File "/opt/conda-src/conda/cli/conda_argparse.py", line 167, in do_call
        result = getattr(module, func_name)(args, parser)
      File "/opt/conda-src/conda/notices/core.py", line 124, in wrapper
        return func(*args, **kwargs)
      File "/opt/conda-src/conda/cli/main_create.py", line 50, in execute
        install(args, parser, "create")
      File "/opt/conda-src/conda/cli/install.py", line 316, in install
        unlink_link_transaction = solver.solve_for_transaction(
      File "/opt/conda-src/conda/core/solve.py", line 154, in solve_for_transaction
        unlink_precs, link_precs = self.solve_for_diff(
      File "/opt/conda-src/conda/core/solve.py", line 215, in solve_for_diff
        final_precs = self.solve_final_state(
      File "/opt/conda-libmamba-solver-src/conda_libmamba_solver/solver.py", line 199, in solve_final_state
        index = IndexHelper(
      File "/opt/conda-libmamba-solver-src/conda_libmamba_solver/index.py", line 125, in __init__
        self._index = self._load_channels()
      File "/opt/conda-libmamba-solver-src/conda_libmamba_solver/index.py", line 285, in _load_channels
        set_channel_priorities(index)
      File "/opt/conda-libmamba-solver-src/conda_libmamba_solver/mamba_utils.py", line 52, in set_channel_priorities
        current_channel = next(iter(index.values())).channel.canonical_name
    StopIteration

because we assume index will always be populated, but it's not in this case.

The fix is to return index early if empty, and we obtain the desired behaviour:

Channels:
 - local
Platform: linux-aarch64
Collecting package metadata (repodata.json): done
Solving environment: failed

PackagesNotFoundError: The following packages are not available from current channels:

  - python

Current channels:

  - local

To search for alternate channels that may provide the conda package you're
looking for, navigate to

    https://anaconda.org

and use the search bar at the top of the page.

This was detected in conda/conda#12984

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jaimergp]

Failures are unrelated (see #242 and conda/conda#12986).

Ready for review @costrouc!

[costrouc]

LGTM!