Use null coalescing operator in WP_Theme_JSON class methods

[mukeshpanchal27]

Trac ticket: https://core.trac.wordpress.org/ticket/63755

This PR makes a minor update to the way default values are assigned for custom template properties in the get_custom_templates method, using the null coalescing operator for improved readability and consistency.

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[github-actions]

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[github-actions]

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Props mukesh27, westonruter.

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[westonruter]

Is Core-63755 the right ticket? This doesn't seem to be a performance improvement.

[mukeshpanchal27]

Closing in favour of #4886