[bug] Previewing an announcement template without filling form field breaks javascript

[KashifSaadat]

Description: When creating an announcement template with optional/required fields, if you attempt to preview the template whilst leaving the form fields blank it causes the page to be in an endless loading phase (and fails to show the preview).

Steps to recreate:

1. Login to Platform Hub as an administrator
2. Navigate to Announcement Templates in the left navigation bar
3. Select NEW in the top right to create a template
4. Fill in all the required sections as normal
5. Select the ADD NEW FIELD button and specify a field with any type, leaving the required box unticked (with the select field, it can break both ways if you tick as required or leave optional)
6. Select the OPEN PREVIEWER button at the bottom
7. Leave the field empty and press the PREVIEW button

Expected results:

• Required Field: The form should prevent you from submitting until a value has been specified
• Optional Field: The announcement template should be rendered successfully

Actual results:
The loading attribute is added to the popup element eternally, and nothing renders. The console shows the following javascript error:

angular.js:14199 Error: "data" argument not specified or empty
    at Object.previewAnnouncementTemplate (hub-api.service.js:422)
    at exports.AnnouncementTemplatePreviewPopupController.preview (announcement-template-preview-popup.controller.js:35)
    at fn (eval at compile (angular.js:15126), <anonymous>:4:271)
    at expensiveCheckFn (angular.js:16213)
    at callback (angular.js:26592)
    at Scope.$eval (angular.js:17994)
    at Scope.$apply (angular.js:18094)
    at HTMLButtonElement.<anonymous> (angular.js:26597)
    at defaultHandlerWrapper (angular.js:3546)
    at HTMLButtonElement.eventHandler (angular.js:3534)