Only open help page for commands with DocLink

The help-command now handles commands without a DocLink
in the same manner as unknown commands.

Fixes #7065

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

@@ -102,7 +102,8 @@ namespace Microsoft.DotNet.Tools.Help
         {
             if (BuiltInCommandsCatalog.Commands.TryGetValue(
                 _appliedOption.Arguments.Single(),
-                out BuiltInCommandMetadata builtIn))
+                out BuiltInCommandMetadata builtIn) &&
+                !string.IsNullOrEmpty(builtIn.DocLink))
             {
                 var process = ConfigureProcess(builtIn.DocLink);
                 process.Start();