Upgrade/Install: Only check plugins whose data is already stored.

[costdev]

In r57592, some update_option() calls were removed from bootstrapping. However, this also removed a check to ensure an array key existed, and populated it if not.

Scaffolding tests by WP-CLI revealed that a plugin in the active_plugins option may not have data already stored within the plugin_data option, causing a PHP warning for an undefined array key.

This adds a condition to ensure the requirements checks are only performed on plugins whose data is already stored in the plugin_data option.

Trac ticket: https://core.trac.wordpress.org/ticket/60461

[github-actions]

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[github-actions]

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[hellofromtonya]

LGTM 👍 Ready for commit.

[costdev]

Committed in 57622.