Fix false update notice when product_version is not yet set

[shazzad]

Summary

• product_version is only populated during the init hook, but pre_set_site_transient_update_plugins can fire earlier (e.g. during plugin activation via refresh_updates_transient/clear_updates_transient)
• With product_version as null, version_compare('5.0.0', null, '>') always returns true, showing an update available even when the installed version matches the API version
• Fix: fall back to the version from $transient->checked (which WordPress populates with installed versions) and bail out early when no version is available

Test plan

• Verify plugin with version 5.0.0 no longer shows "update available" when API also reports 5.0.0
• Verify genuine updates (API version > installed) still show correctly
• Verify plugin activation/deactivation cycle doesn't produce false update notices

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