Darc commands fail with UriFormatException unless I specify `--bar-uri`

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

This repro is on `Microsoft.DotNet.Darc 1.1.0-beta.26374.1+ea4c017203f17b7bbad86bfd19f61e4d34284dc2` but I have tried older versions back to `Microsoft.DotNet.Darc 1.1.0-beta.26359.1` and it repros there as well.

` darc get-channels`

> Unhandled exception. System.UriFormatException: Invalid URI: The URI is empty.
> at System.Uri.CreateThis(String uri, Boolean dontEscape, UriKind uriKind, UriCreationOptions& creationOptions)
> at System.Uri..ctor(String uriString)
> at Microsoft.DotNet.ProductConstructionService.Client.PcsApiFactory.GetAnonymous() in /_/src/ProductConstructionService/Microsoft.DotNet.ProductConstructionService.Client/PcsApiFactory.cs:line 43
> at Microsoft.DotNet.Darc.DarcVersionValidator.ValidateAsync(String baseUri, ILogger logger) in /_/src/Microsoft.DotNet.Darc/Darc/DarcVersionValidator.cs:line 29
> at Microsoft.DotNet.Darc.Program.<>c.<b__0_1>d.MoveNext() in /_/src/Microsoft.DotNet.Darc/Darc/Program.cs:line 58
> --- End of stack trace from previous location ---
> at Microsoft.DotNet.Darc.Program.Main(String[] args) in /_/src/Microsoft.DotNet.Darc/Darc/Program.cs:line 48
> at Microsoft.DotNet.Darc.Program.(String[] args)

Providing `--bar-uri` resolves the issue. (eg, `darc get-channels --bar-uri https://maestro.dot.net/`)
I have to provide this on all commands, even `darc authenticate`.

Start with Microsoft.DotNet.Darc/Darc/Program.cs and Microsoft.DotNet.ProductConstructionService.Client/PcsApiFactory.cs, following the validation path shown in the stack trace. Reproduce `darc get-channels` and `darc authenticate` without `--bar-uri`; done means these commands no longer throw UriFormatException when the option is omitted.

Written by the indexing model from the issue text.