Fix PHP fatal error if Add On check returns null

[dparker1005]

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Changes proposed in this Pull Request:

We saw the following fatal error on the plugins/updates page if the site fails to retrieve the list of Add Ons:

An error of type E_ERROR was caused in line 59 of the file /.../wp-content/plugins/paid-memberships-pro/includes/addons.php. Error message: Uncaught TypeError: array_column(): Argument #1 ($array) must be of type array, null given in /.../wp-content/plugins/paid-memberships-pro/includes/addons.php:59 Stack trace: #0 /.../wp-content/plugins/paid-memberships-pro/includes/addons.php(59): array_column() #1 /.../wp-content/plugins/paid-memberships-pro/includes/addons.php(265): pmpro_getAddons() #2 /.../wp-includes/class-wp-hook.php(326): pmpro_update_plugins_filter() #3 ......

This PR checks whether $addons is empty to avoid passing null into array_column().

Other information:

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