SIGSEGV when attempting to reading i=84 against simulator.

[Scarjit]

While attempting to use our benthos-opcua plugin, we noticed that it can throw a segmentation fault, due to a nil pointer deref, inside the variant.go.
We managed to produce this by subscribing to i=84 against amine-amaach's simulator.

I will attempt to dig into this myself when i have time, and create a PR when i found the fix.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x4 pc=0x28ce9e9]

goroutine 7 [running]:
github.com/gopcua/opcua/ua.(*Variant).ArrayLength(...)
	/go/pkg/mod/github.com/gopcua/[email protected]/ua/variant.go:104
github.com/gopcua/opcua/ua.(*Variant).NodeID(...)
	/go/pkg/mod/github.com/gopcua/[email protected]/ua/variant.go:691
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.browse({0x3b862d8?, 0xc00146a190}, 0xc0015462e0, {0x0, 0x0}, 0x3, 0xc000c493f0, {0xc00190ee80, 0x4})
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:130 +0x8c9
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.browse.func2(0xf7c70?)
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:186 +0x25a
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.browse({0x3b862d8?, 0xc00146a190}, 0xc0016add30, {0x0, 0x0}, 0x2, 0xc000c493f0, {0xc00190ee80, 0x4})
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:221 +0xe02
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.browse.func2(0xf7c70?)
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:186 +0x25a
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.browse({0x3b862d8?, 0xc00146a190}, 0xc001910a70, {0x0, 0x0}, 0x1, 0xc000c493f0, {0xc00190ee80, 0x4})
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:221 +0xe02
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.browse.func2(0xf7c70?)
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:186 +0x25a
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.browse({0x3b862d8?, 0xc00146a190}, 0xc001910500, {0x0, 0x0}, 0x0, 0xc000c493f0, {0xc00190ee80, 0x4})
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:221 +0xe02
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.(*OPCUAInput).BrowseAndSubscribeIfNeeded(0xc000ebe6c0, {0x3b862d8, 0xc00146a190})
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:866 +0x1f4
github.com/united-manufacturing-hub/benthos-umh/v2/opcua_plugin.(*OPCUAInput).Connect(0xc000ebe6c0, {0x3b862d8, 0xc00146a190})
	/go/src/github.com/united-manufacturing-hub/benthos-umh/opcua_plugin/opcua.go:844 +0x12cc
github.com/benthosdev/benthos/v4/public/service.(*autoRetryInputBatched).Connect(0x10?, {0x3b862d8?, 0xc00146a190?})
	/go/pkg/mod/github.com/benthosdev/benthos/[email protected]/public/service/input_auto_retry_batched.go:52 +0x26
github.com/benthosdev/benthos/v4/public/service.(*airGapBatchReader).Connect(0x30?, {0x3b862d8?, 0xc00146a190?})
	/go/pkg/mod/github.com/benthosdev/benthos/[email protected]/public/service/input.go:150 +0x25
github.com/benthosdev/benthos/v4/internal/component/input.(*AsyncReader).loop.func3()
	/go/pkg/mod/github.com/benthosdev/benthos/[email protected]/internal/component/input/async_reader.go:99 +0x66
github.com/benthosdev/benthos/v4/internal/component/input.(*AsyncReader).loop(0xc0009bf1a0)
	/go/pkg/mod/github.com/benthosdev/benthos/[email protected]/internal/component/input/async_reader.go:116 +0x37a
created by github.com/benthosdev/benthos/v4/internal/component/input.NewAsyncReader in goroutine 1
	/go/pkg/mod/github.com/benthosdev/benthos/[email protected]/internal/component/input/async_reader.go:57 +0x245