Admin app fails to update partners if existing document is malformed

[dianabarsan]

Describe the bug
A malformed partners doc will block the user from updating partners from the admin app.

To Reproduce
Steps to reproduce the behavior:

1. Create a partners.json document in your config. Set the contents of this document to be {}. run cht upload-partners.
2. Login to admin app. Go to Images -> Partners.
3. Add a new "partner".
4. Click Submit.
5. See error There was an error saving your settings, please try again..

Expected behavior
The UI should inform the user that the existing document is malformed.
Even when the document is malformed, the user should have the possibility to "fix" it by editing from the Admin UI.

Logs
When loading the partners page:

angular.js:15697 TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at resource-icons.js:111:24
    at processQueue (angular.js:18075:1)
    at angular.js:18123:1
    at Scope.$digest (angular.js:19242:1)
    at angular.js:19562:1
    at TaskTracker.completeTask (angular.js:21403:1)
    at angular.js:6879:1 'Possibly unhandled rejection: {}'

When saving:

angular.js:15697 Error uploading image TypeError: Cannot set properties of undefined (setting 'partner-name')
    at images-partners.js:54:27
    at processQueue (angular.js:18075:1)
    at angular.js:18123:1
    at Scope.$digest (angular.js:19242:1)
    at angular.js:19562:1
    at TaskTracker.completeTask (angular.js:21403:1)
    at angular.js:6879:1

Environment

• Instance: any
• App: admin
• Version: 3.4.0+

Additional context
Form post