`plots diff` & `exp show`: unexpected error - Empty git repo

Bug Report

Description

After initialising a repository with both Git and DVC both plots diff and exp show throw ERROR: unexpected error - Empty git repo.

Reproduce

1. mkdir new
2. cd new
3. git init
4. dvc init
5. dvc exp show => throws error
6. dvc plots diff => throws error

Expected

Neither of these commands should throws an error under these circumstances but they should return an empty dict for the --json option.

Environment information

Output of dvc doctor:

$ dvc doctor
DVC version: 2.18.1 (pip)
---------------------------------
Platform: Python 3.9.13 on macOS-12.5.1-arm64-arm-64bit
Supports:
	http (aiohttp = 3.8.1, aiohttp-retry = 2.8.3),
	https (aiohttp = 3.8.1, aiohttp-retry = 2.8.3),
	webhdfs (fsspec = 2022.7.1)
Cache types: <https://error.dvc.org/no-dvc-cache>
Caches: local
Remotes: None
Workspace directory: apfs on /dev/disk3s1s1
Repo: dvc, git

Additional Information (if any):

This was brought up in the cross team meeting and is part of a larger discussion on error handling.

The requested behaviour is exhibited under the same circumstances for the following commands:

/new master +3 ❯ dvc list . --dvc-only --recursive --json
[]
/new master +3 ❯ dvc status --json
{}
/new master +3 ❯ dvc diff --json
{}
/new master +3 ❯ dvc data status --json
{}

[pmrowla]

https://github.com/iterative/dvc/blob/460d08057cf355686e1337b94640ac8522497bb1/dvc/scm.py#L163

This should also be

if not (rev == "HEAD" or rev.startswith("refs/")):

(This does not resolve the issue, but it makes DVC generate a proper error message rather than unexpected error.)

ERROR: failed to show experiments - unknown Git revision ‘HEAD’

[daavoo]

I think we just need to do the same as dvc diff / exp diff / metrics diff / params diff:

https://github.com/iterative/dvc/blob/460d08057cf355686e1337b94640ac8522497bb1/dvc/repo/diff.py#L23-L24