Deleting everything from Web Content structure Source throws "Syntax error: Unexpected end of JSON Input"

• Type: Bug
• Status: Closed
• Resolution: Fixed
• Affects Version/s: Master
• Fix Version/s: 7.0.0 DXP SP2, 7.0.0 DXP FP13, 7.0.0 DXP SP3, 7.0.3 CE GA4, 7.1.X, Master
• Component/s: Dynamic Data Mapping, Dynamic Data Mapping > Form Builder
• Labels:
  • fw_code_review
  • fw_test_manual
  • liferay-digital-enterprise-70-sp3
  • liferay-fixpack-de-13-7010
• Branch Version/s:
  7.0.x
• Backported to Branch:
  Committed
• Story Points: 1.25
• Fix Priority: 2
• Git Pull Request: https://github.com/liferay/com-liferay-dynamic-data-mapping/pull/38

Description
When editing a Web Content Stucture's JSON in the Source tab, if you delete everything or delete the fields element, errors are thrown.

Steps to Reproduce
1. Go to Site Content -> Web Content -> (kebab) -> Structures
2. Add a structure
3. Add an element to the Structure in the View tab (ex. a Date)
4. Go over to the Source Tab and delete all contents of it.
5. Go back to the View Tab.

Expected Result: No errors are thrown in the browser's console
Actual Result: An error is thrown, Uncaught SyntaxError: Unexpected end of JSON input.

6. Refresh the page and return to the Source tab
7. Delete the Fields element (only availableLanguageId and defaultLanguageId should be left).

Expected Result: No errors are thrown in the console
Actual Result: Uncaught TypeError: Cannot read property 'forEach' of undefined is thrown in the console.