Generate our own senderkey if it does not exist

This case can happen on restore from seed

ts/receiver/mediumGroups.ts

@@ -32,14 +32,20 @@ async function handleSenderKeyRequest(
 
   log.debug('[sender key] sender key request from:', senderIdentity);
 
-  const maybeKey = await getChainKey(groupId, ourIdentity);
+  let maybeKey = await getChainKey(groupId, ourIdentity);
 
   if (!maybeKey) {
-    // Regenerate? This should never happen though
-    log.error('Could not find own sender key');
+    log.error('Could not find own sender key. Generating new one.');
+    maybeKey = await SenderKeyAPI.createSenderKeyForGroup(
+      groupId,
+      PubKey.cast(ourIdentity)
+    );
+    if (!maybeKey) {
+      log.error('Could not find own sender key after regenerate');
       await removeFromCache(envelope);
       return;
     }
+  }
 
   // We reuse the same message type for sender keys
   const { chainKey, keyIdx } = maybeKey;