OfflineContentManager export_items fails for "Tile layer (hosted)" map service

**Describe the bug**
When using the OfflineContentManager's export_items method with a "Tile layer (hosted)" map service, published from a Tile Package, it unexpectedly fails. (See error message below.)

**To Reproduce**
In order to reproduce the issue you can attempt to use export_items with a "Tile layer (hosted)" map service, published from a Tile package. It will result in the following error:

```python
C:\Users\knoop\AppData\Local\ESRI\conda\envs\work\Lib\site-packages\arcgis\apps\itemgraph\_migration.py:149: RuntimeWarning: Failed to export item 9ad056d339534fdb9c75fa354491f101 due to error: is not JSON serializable. Deleting folder and skipping...
warnings.warn(
```

**Expected behavior**
I expect export_items to be able to successfully export a "Tile layer (hosted)" map service, including its Tile package.

**Platform (please complete the following information):**
Python API 2.4.2