Don't dereference a null type when building a template argument name

[tannergooding]

Fixes #580.

Generation crashes with a NullReferenceException while building the qualified name of a class template specialization:

System.NullReferenceException: Object reference not set to an instance of an object.
   at ClangSharp.PInvokeGenerator.<GetCursorQualifiedName>g__AppendTemplateArgument|...
   at ClangSharp.PInvokeGenerator.<GetCursorQualifiedName>g__AppendTemplateArguments|...
   at ClangSharp.PInvokeGenerator.GetCursorQualifiedName(NamedDecl, Boolean)
   at ClangSharp.PInvokeGenerator.<VisitTypedefDecl>g__ForUnderlyingType|...

Root cause. AppendTemplateArgument handles a Type-kind argument with templateArgument.AsType.AsString. AsType routes through TranslationUnit.GetOrCreate<Type>, which returns null for a CXType_Invalid handle (the Debug.Assert guarding that path is compiled out of Release). So when an argument's underlying QualType maps to an invalid CXType — an unsupported/exotic type the shim can't represent, as happens deep in the sol2 / spdlog / C++20 template instantiations from the report — AsType is null and .AsString throws. AsIntegral returns a long and the default branch is already safe, so this is the only unguarded deref.

Fix. Guard the access and fall back to the same '?' placeholder the default branch already uses for unsupported argument kinds:

var argType = templateArgument.AsType;
_ = qualifiedName.Append((argType is not null) ? argType.AsString : ""?"");

The success path is byte-identical (non-null AsType still yields AsType.AsString); the only new behavior is that a previously-fatal NRE now renders as Foo<?> in the qualified name (used for diagnostics and remapping keys), matching how unsupported kinds already render.

On testing. I verified the mechanism directly from source and confirmed it's the only null-deref in the method. I could not build a minimal standalone repro — the invalid-CXType argument only arises from the specific deeply-nested standard-library instantiations in the original report, which need the full ModEngine2 + sol2 + spdlog include tree and toolchain to stand up. This mirrors the earlier harden generator against malformed inputs change, which likewise guards a native-only crash path that isn't expressible through the golden-file harness. Full suite green with zero golden changes.

Note

This PR description and the code change were drafted by Copilot on my behalf.