triple-slash links to .NET base libraries erase output

[Emdot]

Operating System: Windows

DocFX Version Used: 2.56.7.0

Template used: default

Steps to Reproduce

Here's a minimal repro: docfx-issue.zip

1. In C# code, add /// comments that reference a BCL class, such as System.Exception.
2. Add all relevant using lines.
3. In docfx.json, set xrefService to https://xref.docs.microsoft.com/query?uid={uid}
4. Run docfx

Expected Behavior

Ideally, docfx should use the xrefService to make a link. At minimum, though, it should show the original text.

Actual Behavior

docfx fails to resolve the links, and the text does not appear.

Details

The C# from the repro:

using System;
using System.Threading.Tasks;
namespace Testing
{
	public class Class1 
	{ 
		/// <summary>
		/// Cref: <see cref="System.Threading.Tasks.Task"/>
		/// <br/>
		/// Xref: @System.Threading.Tasks.Task
		/// </summary>
		/// <exception cref="System.Exception">exception doc</exception>
		public void Foo()
		{
		}
	}
}

In the output, note that:

• The xref resolved correctly, which shows that docfx can resolve the reference and that xrefService is configured correctly.
• For the cref, the text is missing. At minimum, it should say System.Threading.Tasks.Task even if it's not a link.
• The exception documentation is missing entirely.
  

These are the warnings:

[21-03-30 02:09:08.783]Warning:[MetadataCommand.ExtractMetadata]Invalid cref value "!:System.Threading.Tasks.Task" found in triple-slash-comments for Foo defined in C:\Users\3mave\source\tests\docfx-issue\src\Class1.cs Line 12, ignored.
[21-03-30 02:09:08.786]Warning:[MetadataCommand.ExtractMetadata]Invalid cref value "!:System.Exception" found in triple-slash-comments for Foo defined in C:\Users\3mave\source\tests\docfx-issue\src\Class1.cs Line 12, ignored.

[Emdot]

If you can point me to the right place in the code, I'm willing to submit a PR (assuming it's not especially complicated).

[Algorithman]

Made a PR, it replaces the unresolved ref with the cref value, still printing the warnings, but doesn't leave doc's without the class/member names

[Emdot]

For other readers: the PR is #7325