Prune to_device messages to avoid sending empty messages

Fixes https://github.com/vector-im/riot-web/issues/13322

src/crypto/algorithms/megolm.js

@@ -521,6 +521,33 @@ MegolmEncryption.prototype._encryptAndSendKeysToDevices = function(
     }
 
     return Promise.all(promises).then(() => {
+        // prune out any devices that encryptMessageForDevice could not encrypt for,
+        // in which case it will have just not added anything to the ciphertext object.
+        // There's no point sending messages to devices if we couldn't encrypt to them,
+        // since that's effectively a blank message.
+        for (const userId of Object.keys(contentMap)) {
+            for (const deviceId of Object.keys(contentMap[userId])) {
+                if (Object.keys(contentMap[userId][deviceId].ciphertext).length === 0) {
+                    logger.log(
+                        "No ciphertext for device " +
+                        userId + ":" + deviceId + ": pruning",
+                    );
+                    delete contentMap[userId][deviceId];
+                }
+            }
+            // No devices left for that user? Strip that too.
+            if (Object.keys(contentMap[userId]).length === 0) {
+                logger.log("Pruned all devices for user " + userId);
+                delete contentMap[userId];
+            }
+        }
+
+        // Is there anything left?
+        if (Object.keys(contentMap).length === 0) {
+            logger.log("No users left to send to: aborting");
+            return;
+        }
+
         return this._baseApis.sendToDevice("m.room.encrypted", contentMap).then(() => {
             // store that we successfully uploaded the keys of the current slice
             for (const userId of Object.keys(contentMap)) {