Customize: Support hiding random default headers

[ArkaPrabhaChowdhury]

Trac ticket: https://core.trac.wordpress.org/ticket/46128

When a theme enables random default header images without setting a fixed default image, the Customizer currently treats the header image setting as empty. This hides the Hide image button even though a randomized header is displayed.

This sets the Customizer setting default to random-default-image when the theme enables random default headers, while preserving the existing fixed default behavior otherwise. Regression coverage verifies both cases.

Testing

• npm run test:php -- --filter Tests_WP_Customize_Manager::test_header_image_setting_default
• PHPCS for both changed files
• PHP syntax checks for both changed files

[github-actions]

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