Form Builder: Check for Access Token

[n7studios]

Summary

Fixes this reported issue by showing the user they need to connect their Kit account before using the Form Builder block, in the same way the Plugin shows this across other blocks (Form, Product, Broadcasts etc).

Testing

• testFormBuilderBlockWhenNoCredentials: Test that the expected message displays instead of a JS error, and connecting results in the block working correctly.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)