Reject nbgv get-version ""

[KalleOlaviNiemitalo]

I'd like nbgv get-version "" to return an error, like git tag does:

$ git tag hah ""
fatal: Failed to resolve '' as a valid ref.

In nbgv 3.5.108+6e793d63d3, both nbgv get-version and nbgv get-version "" mean the same as nbgv get-version HEAD.

Rationale

To work around nbgv not supporting all the same <commit-ish> syntax as Git, my script preprocessed the argument like nbgv get-version "$(git rev-parse "${rev}")". But when ${rev} was accidentally empty, the script executed nbgv get-version "", which displayed the version information of HEAD instead. If that had returned an error right away, it would have finished faster (nbgv get-version HEAD takes 1500 ms in my repo) and not output misleading information.

Implementation ideas

nbgv treats an empty string as equivalent to null here:

Nerdbank.GitVersioning/src/nbgv/Program.cs

Lines 382 to 385 in 6e793d6

	if (string.IsNullOrEmpty(commitish))
	{
	commitish = DefaultRef;
	}

Could either tighten that check, or add a validator for the argument here:

Nerdbank.GitVersioning/src/nbgv/Program.cs

Lines 114 to 117 in 6e793d6

	new Argument<string>("commit-ish", () => DefaultRef, $"The commit/ref to get the version information for.")
	{
	Arity = ArgumentArity.ZeroOrOne,
	},

[AArnott]

I suppose nbgv get-version "" can be made to error out, provided nbgv get-version still works.
I'd favor rejecting "" at the command parsing level rather than in the command handler.