Simplify code for ProtoAdapters of types with no fields

Example:

companion object {
    @JvmField
    val ADAPTER: ProtoAdapter<NodeEvent> = object : ProtoAdapter<NodeEvent>(
        FieldEncoding.LENGTH_DELIMITED,
        NodeEvent::class.java
    ) {
        override fun decode(reader: ProtoReader): NodeEvent {
            val unknownFields = reader.forEachTag { tag ->
                when (tag) {
                    else -> TagHandler.UNKNOWN_TAG
                }
            }
   ...
}

The when block is unnecessary, since NodeEvent doesn't have any fields. This can be simplified to:

val unknownFields = reader.forEachTag { TagHandler.UNKNOWN_TAG }

Locate the ProtoAdapter generation for types with no fields and compare it with the NodeEvent example in the issue. Check existing generated-code tests or fixtures for empty types, then verify that the generated adapter uses the simplified tag handler and that the relevant tests still pass.