Change the return type of `_get_cron_array()` to an empty array if the `cron` option is either missing or of an
unexpected type.
This change ensures the return value for no registered events is consistently an empty array. Previously the return
value could be either an empty array or `false`.
Props thakkarhardik, jrf, costdev.
Fixes#53940.
git-svn-id: https://develop.svn.wordpress.org/trunk@53791 602fd350-edb4-49c9-b593-d223f7449a82
