fix(gnovm): allow type-switch with sole case nil: (preprocess single-case branch must tag-type-only when ct is nil)

[ltzmaxwell]

• Before: a type-switch whose sole case was case nil: (e.g. switch xx := x.(type) { case nil: ... }) panicked at preprocess with name xx not declared because the single-case shortcut registered the switch var with anyValue(ct) where ct == nil; after: the sole-nil form is accepted and xx is bound to the tag's static type, matching Go.
• preprocess1's *TypeSwitchStmt branch in gnovm/pkg/gnolang/preprocess.go now gates the len(n.Cases) == 1 define-with-case-type shortcut on ct != nil, so a nil ct falls through to the same tag-type define used for multi-case switches.
• New filetest gnovm/tests/files/typeswitch1.gno mirrors gocorpus's typeswitch1 (sole case nil: alongside mixed multi-type cases) and exercises both the xx := x.(type) and bare xx.(type) shapes.

[Gno2D2]

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[notJoon]

The change look good. I left a minor suggestion; please take a look and feel free to apply them.

not necessary, but I think it would be good to have a test that distinguishes between an interface containing a nil value and one with no dynamic type at all.

for example:

// PATH: gnovm/tests/files/typeswitch2.gno

package main

type S struct{}

func classify(x any) {
	switch v := x.(type) {
	case nil:
		println("nil", v == nil)
	case *S:
		println("*S", v == nil)
	default:
		panic("unexpected type")
	}
}

func main() {
	classify(nil)
	classify((*S)(nil))
	classify(&S{})
}

// Output:
// nil true
// *S true
// *S false

https://go.dev/play/p/hOcVkHTayGI

nil has no dynamic type, so it enters case nil. In contrast, (*S)(nil) still has the dynamic type *S even though the underlying pointer value is nil, so it enters case *S. and I confirmed that this passes on my side.

Since this change adds support for case nil, testing this boundary as well would help ensure the behavior is robust and the intent is clear.

[ltzmaxwell]

Added the suggested typeswitch2.gno filetest in 47d220b — good boundary to pin down: nil (no dynamic type) enters case nil, while (*S)(nil) keeps dynamic type *S and enters case *S with a nil underlying value. Passes locally. Thanks @notJoon!