internal/pipewire: handle nil spa_dict correctly

This now marshals into a value of type None when the slice is nil, and correctly unmarshals from type None.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pipewire/pod.go

@@ -541,6 +541,11 @@ func (d *SPADict) Size() Word {
 // MarshalPOD satisfies [PODMarshaler] as [SPADict] violates the POD type system.
 func (d *SPADict) MarshalPOD(data []byte) ([]byte, error) {
 	return appendInner(data, func(dataPrefix []byte) (data []byte, err error) {
+		if *d == nil {
+			data = SPA_TYPE_None.append(dataPrefix)
+			return
+		}
+
 		data = SPA_TYPE_Struct.append(dataPrefix)
 		if data, err = MarshalAppend(data, Int(len(*d))); err != nil {
 			return
@@ -560,6 +565,14 @@ func (d *SPADict) MarshalPOD(data []byte) ([]byte, error) {
 // UnmarshalPOD satisfies [PODUnmarshaler] as [SPADict] violates the POD type system.
 func (d *SPADict) UnmarshalPOD(data []byte) (Word, error) {
 	var wireSize Word
+
+	if ok, err := unmarshalHandleNone(&data, &wireSize); err != nil {
+		return wireSize, err
+	} else if ok {
+		*d = nil
+		return wireSize, nil
+	}
+
 	if err := unmarshalCheckTypeBounds(&data, SPA_TYPE_Struct, &wireSize); err != nil {
 		return wireSize, err
 	}