A blank SeeAlso or Exceptions cref, or a whitespace InheritDocCref, writes cref="" (CS1584), and a null SeeAlso entry throws NullReferenceException and leaves the class without that member #208

What's wrong

DocComment treats an empty InheritDocCref as "no cref" and writes a bare <inheritdoc/> (CodeBlocker/Templates/DocComment.cs:115). That is the only place blank crefs are handled:

• Blank crefs are written as-is. A whitespace InheritDocCref, a blank or whitespace SeeAlso entry, and a blank Exceptions[i].Name are each written as cref="" or cref=" " by the loops at DocComment.cs:135-151. ValidateAttributes (:200-222) checks these values only for line breaks.
• A null entry throws mid-write. A null SeeAlso entry passes ValidateAttributes, then throws a NullReferenceException in EscapeAttribute while the doc comment is being written. By then the class has already been partly written. The same applies to a DocTag whose Name is null.

Elsewhere the library skips blank list entries (TemplateRendering.NonBlank, #197) and refuses null ones before writing anything (#198/#199). These two lists do neither.

Repro (HEAD 032f69a)

var d = new DocComment { InheritDoc = true, InheritDocCref = "  " };
d.SeeAlso.Add(""); d.SeeAlso.Add("  ");
d.Exceptions.Add(new DocTag { Name = "", Text = "x" });
c.Members.Add(new MethodTemplate { Name = "M", Type = "void", Documentation = d, BodyFactory = ... });
cb.AddClass(c);

class C
{
	/// <inheritdoc cref="  "/>
	/// <exception cref="">x</exception>
	/// <seealso cref=""/>
	/// <seealso cref="  "/>
	void M()
	{
	}
}

When this output is compiled with GenerateDocumentationFile, each of those lines reports warning CS1584: XML comment has syntactically incorrect cref attribute '' plus CS1658. Under TreatWarningsAsErrors they become build errors.

With d.SeeAlso.Add(null!) (Summary = "hi"), AddClass throws NullReferenceException and leaves:

class C
{
	/// <summary>hi</summary>
}

Why it matters

Generators build these lists from data, such as the exceptions a source method documents or "see also" links taken from metadata. A missing or blank value there turns the generated project's build red. A null value leaves a corrupt partial output and a bare NullReferenceException, which is the #164/#198 symptom again.

Suggested fix / acceptance criteria

• Skip null or whitespace SeeAlso entries and Exceptions entries whose Name is blank, as NonBlank does for other lists.
• Treat a whitespace InheritDocCref like an empty one, and write a bare <inheritdoc/>.
• If null entries should be rejected rather than skipped, have ValidateAttributes refuse a null DocTag and a null SeeAlso entry or Name before anything is written, matching Refuse a null parameter, member or nested type before writing [patch] #199.
• Add tests: blank and whitespace crefs produce no cref="" lines, and a null entry either is skipped or throws before any output is written.