[Nextjs] Application build fails due to missing component variants

[illiakovalenko]

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

Description / Motivation

Added a missing check in placeholder utils to ensure the MissingComponent is rendered when a default or custom variant is referenced in the layout tree but not implemented or exported by the component.
Added a minor improvement to log a missing variant as well

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)

[github-actions]

📦 Package Size and Test Coverage Report

Package	Base Size	PR Size	Δ Change	Base Coverage	PR Coverage	Δ Change
cli	56.62 KB	56.62 KB	✅ 0.00 KB	69.73%	69.73%	0.00%
core	484.08 KB	484.08 KB	✅ 0.00 KB	90.51%	90.51%	0.00%
create-content-sdk-app	234.46 KB	234.46 KB	✅ 0.00 KB	96.22%	96.22%	0.00%
nextjs	360.31 KB	360.31 KB	✅ 0.00 KB	91.54%	91.54%	0.00%
react	214.74 KB	215.75 KB	🔺 +1.01 KB	92.30%	92.30%	0.00%
Total	—	—	🔺 +1.01 KB	—	—	—