Site sign failed when there is no files_optional

Got the following error when there is no `"files_optional": {}` node in `content.json` (this was a newly created site and I want to upload something to it):
```
Site sign failed: content.json: TypeError: argument of type 'NoneType' is not iterable in UiWebsocket.py line 382 > ContentManager.py line 577 > ContentManager.py line 524 > BigfilePlugin.py line 244
```

Start at BigfilePlugin.py line 244 and follow the call chain through ContentManager.py lines 524 and 577 to UiWebsocket.py line 382. Reproduce signing or uploading a newly created site whose content.json omits files_optional, then verify the operation no longer raises the reported TypeError.

Written by the indexing model from the issue text.