Options API: Fix delete_option() cache cleanup for null values

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/28701

This PR fixes a long-standing bug where delete_option() fails to remove options with null values from the alloptions cache.

When an autoloaded option contains null, delete_option() uses isset($alloptions[$option]) to check cache presence. Since isset(null) === false, options with null values remain in cache after deletion, causing get_option() to return stale cached values instead of defaults.

This PR -

• Replaces isset() with array_key_exists() for proper null detection
• Always calls wp_cache_delete() to ensure complete cache cleanup
• Adds a unit test to prevent regression

[github-actions]

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