[MXHTTPClient] cleanupBackgroundTask keyNotFound(CodingKeys(stringValue: "msgtype", intValue: nil)

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**Describe the bug**
During the sync of our E2E encrypted mobile client we notice an error message in console output:

`[MXHTTPClient] cleanupBackgroundTask
keyNotFound(CodingKeys(stringValue: "msgtype", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key CodingKeys(stringValue: \"msgtype\", intValue: nil) (\"msgtype\").", underlyingError: nil))`

full log

```
2021-10-26 12:33:51.085697+0200 [2751:509035] [MXHTTPClient] #70 - GET _matrix/client/r0/sync
2021-10-26 12:33:51.090745+0200 [2751:514742] ⚠️ [MXRoomLastMessage] encryptData: no key method provided for encryption.
2021-10-26 12:33:51.096359+0200 [2751:509035] [MXFileStore commit] lasted 19ms
2021-10-26 12:33:51.096498+0200 [2751:509035] [MXSyncResponseFileStore] readMetaData: File does not exist
2021-10-26 12:33:51.096587+0200 [2751:509035] [MXSyncResponseFileStore] readMetaData: File does not exist
2021-10-26 12:33:51.096725+0200 [2751:514742] [MXSyncResponseFileStore] saveMetaData: Remove file
2021-10-26 12:33:51.157806+0200 [2751:509035] [MXHTTPClient] #69 - POST _matrix/client/r0/keys/upload completed in 80ms
2021-10-26 12:33:51.157949+0200 [2751:514768] [MXCrypto] generateAndUploadFallbackKey: Finished uploading fallback key.
2021-10-26 12:33:51.158043+0200 [2751:509035] [MXHTTPClient] cleanupBackgroundTask
keyNotFound(CodingKeys(stringValue: "msgtype", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key CodingKeys(stringValue: \"msgtype\", intValue: nil) (\"msgtype\").", underlyingError: nil))
2021-10-26 12:33:54.824739+0200 [2751:509035] [MXHTTPClient] #70 - GET _matrix/client/r0/sync completed in 3738ms
2021-10-26 12:33:54.826220+0200 [2751:509035] [MXHTTPClient] cleanupBackgroundTask
2021-10-26 12:33:54.827451+0200 [2751:509035] [MXSession] Received sync response in 3742ms
```

**To Reproduce**
Steps to reproduce the behavior:
1. Enable e2e encryption as described in #1254
2. Receive an encrypted file
3. Observe console output

**Expected behavior**
No warning / error about key decoding should be thrown.

**Smartphone (please complete the following information):**
- Device: iPhone 12
- OS: iOS 15.0.2
- Version `v0.20.3`

**Additional context**
E2E Encryption is enabled

Start at MXHTTPClient.cleanupBackgroundTask and trace the response handling reached during _matrix/client/r0/sync and key upload requests. Reproduce the issue by enabling end-to-end encryption and receiving an encrypted file, then verify that the keyNotFound warning no longer appears in console output.

Written by the indexing model from the issue text.