Only include "body" for headers that can have bodies.

briar-headless/src/main/java/org/briarproject/briar/headless/messaging/OutputPrivateMessage.kt

@@ -5,7 +5,7 @@ import org.briarproject.briar.api.messaging.PrivateMessage
 import org.briarproject.briar.api.messaging.PrivateMessageHeader
 import org.briarproject.briar.headless.json.JsonDict
 
-internal fun PrivateMessageHeader.output(contactId: ContactId, body: String?) = JsonDict(
+internal fun PrivateMessageHeader.output(contactId: ContactId) = JsonDict(
     "type" to "PrivateMessage",
     "contactId" to contactId.int,
     "timestamp" to timestamp,
@@ -14,10 +14,15 @@ internal fun PrivateMessageHeader.output(contactId: ContactId, body: String?) =
     "sent" to isSent,
     "local" to isLocal,
     "id" to id.bytes,
-    "groupId" to groupId.bytes,
-    "body" to body
+    "groupId" to groupId.bytes
 )
 
+internal fun PrivateMessageHeader.output(contactId: ContactId, body: String?): JsonDict {
+    val dict = output(contactId)
+    dict["body"] = body
+    return dict
+}
+
 internal fun PrivateMessage.output(contactId: ContactId, body: String) = JsonDict(
     "type" to "PrivateMessage",
     "contactId" to contactId.int,

briar-headless/src/main/java/org/briarproject/briar/headless/messaging/OutputPrivateResponse.kt

@@ -12,7 +12,7 @@ import org.briarproject.briar.api.sharing.InvitationResponse
 import org.briarproject.briar.headless.json.JsonDict
 
 internal fun PrivateResponse.output(contactId: ContactId): JsonDict {
-    val dict = (this as PrivateMessageHeader).output(contactId, null)
+    val dict = (this as PrivateMessageHeader).output(contactId)
     dict.putAll(
         "sessionId" to sessionId.bytes,
         "accepted" to wasAccepted()