Ensure AllowEmptySignList logic in Sign.proj is exercised in Arcade Validation

[missymessa]

There is (existing) error logic surrounding the AllowEmptySignList parameter in Sign.proj. We should ensure that Arcade Validation exercises this logic.

Related PRs:

• Add build manifest #5210

Code that needs testing:

• https://github.com/dotnet/arcade/blob/master/src/Microsoft.DotNet.Arcade.Sdk/tools/Sign.proj