ua: add support for encoding null variants

[ceh]

Follow-up to #284, add a codec test case for a variant with a value of zero and ensure that it can be encoded without yielding a run-time panic.

What do you think @kung-foo, @magiconair?

[ceh]

v.Type() causes a panic when v is the zero value (reflect.ValueOf(nil)). We use the Kind method prior to checking Type here to handle zero value. See https://pkg.go.dev/reflect?tab=doc#Value.

[magiconair]

Which codepath does the test check?

[ceh]

Which codepath does the test check?

Encoding a NULL variant.

The following program:

package main

import (
	"github.com/gopcua/opcua/ua"
)

func main() {
	v := ua.Variant{}
	_, err := v.Decode([]byte{0x0})
	if err != nil {
		panic(err)
	}
	v.Encode()
}

Causes a run-time panic when calling v.Encode() on the current main branch:

panic: reflect: call of reflect.Value.Interface on zero Value

goroutine 1 [running]:
reflect.valueInterface(0x0, 0x0, 0x0, 0x1, 0x8, 0x4eb960)
	/usr/local/go/src/reflect/value.go:1005 +0x1a5
reflect.Value.Interface(...)
	/usr/local/go/src/reflect/value.go:1000
github.com/gopcua/opcua/ua.(*Variant).encode(0xc000135f40, 0xc000135ec8, 0x0, 0x0, 0x0)
	/home/ceh/code/src/github.com/gopcua/opcua/ua/variant.go:327 +0x119
github.com/gopcua/opcua/ua.(*Variant).Encode(0xc000135f40, 0xc0000153e3, 0x1, 0x1, 0x1, 0x0)
	/home/ceh/code/src/github.com/gopcua/opcua/ua/variant.go:312 +0x180
main.main()
	/home/ceh/nullvariant.go:13 +0x89
exit status 2

[magiconair]

But this doesn't cover the sliceDim(nil) case, correct? I'm looking at the PR and see two fixes but only one additional test. I'm not sure if the fixes are related. Can you elaborate?

[ceh]

But this doesn't cover the sliceDim(nil) case, correct? I'm looking at the PR and see two fixes but only one additional test. I'm not sure if the fixes are related. Can you elaborate?

Ah, yeah. That was unclear. The codepath was implicitly tested via the codec test case that uses MustVariant.

I added two additional tests now to make it more explicit, one for sliceDim and one for setting the value of NewVariant.

[magiconair]

Thank you