ESD-34143: Prevent saving branding data in ul customize command if empty

[sergiught]

🔧 Changes

The changes in this PR prevent an error thrown when using the Auth0 CLI to customize the Universal Login, but not providing a page template.

📚 References

🔬 Testing

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (5f72af6) 72.37% compared to head (ce70c5d) 72.32%.

Files	Patch %	Lines
internal/cli/universal_login_customize.go	0.00%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #968      +/-   ##
==========================================
- Coverage   72.37%   72.32%   -0.06%     
==========================================
  Files          93       93              
  Lines       12748    12755       +7     
==========================================
- Hits         9227     9225       -2     
- Misses       2974     2980       +6     
- Partials      547      550       +3     

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