Site Health: Avoid "Undefined property" PHP notices in...

Site Health: Avoid "Undefined property" PHP notices in `wp_dashboard_site_health()` when the status result transient does not exist yet.

Props dlh for initial patch.
See #47606.
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git-svn-id: https://core.svn.wordpress.org/trunk@46869 1a063a9b-81f0-0310-95a4-ce76da25c4cd