internal/pipewire: return correct size for nil spa_dict

A nil spa_dict results in a None type value being sent over the wire.

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

internal/pipewire/pod.go

@@ -505,7 +505,8 @@ type SPADict []SPADictItem
 // Size satisfies [KnownSize] with a value computed at runtime.
 func (d *SPADict) Size() Word {
 	if d == nil {
-		return 0
+		// None type prefix with zero-length body
+		return SizePrefix
 	}
 
 	// struct prefix, NItems value