fix test baseline and handle scenario when no command args are passed

src/Cli/dotnet/commands/dotnet-help/HelpCommand.cs

@@ -16,7 +16,7 @@ namespace Microsoft.DotNet.Tools.Help
 
             result.ShowHelpOrErrorIfAppropriate();
 
-            if (result.GetValue(HelpCommandParser.Argument) is string[] args)
+            if (result.GetValue(HelpCommandParser.Argument) is string[] args && args is not [])
             {
                 return new HelpCommand(args).Execute();
             }

test/dotnet-help.Tests/GivenThatIWantToShowHelpForDotnetHelpCommand.cs

@@ -10,7 +10,7 @@ namespace Microsoft.DotNet.Help.Tests
   .NET CLI help utility
 
 Usage:
-  dotnet help [<COMMAND_NAME>] [options]
+  dotnet help [<COMMAND_NAME>...] [options]
 
 Arguments:
   <COMMAND_NAME>  The SDK command to launch online help for.