[React][nextjs] Do not apply ErrorBoundaries to empty placeholders

[art-alexeyenko]

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added

Description / Motivation

This PR fixes extra markup being generated for empty placeholders, which broke adding components in Experience Editor.
This also does a couple other things:

• Add 'empty placeholder' mock data without 'rendering' code blocks. Empty placeholders would not render them.
• Shorten the code blocks content for empty placeholder data, making it easier to maintain and read
• make all the 'empty placeholder' unit tests use the 'empty placeholder' mock data

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[yavorsk]

👌