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 :warning:

Additional details and impacted files

```diff @@ 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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