Ensure that snapshot data is always an array

[westonruter]

Should fix these issues when data is corrupted:

• Warning: Invalid argument supplied for foreach() in .../customize-snapshots/php/class-customize-snapshot-manager.php on line 753
• Warning: Invalid argument supplied for foreach() in .../customize-snapshots/php/class-customize-snapshot.php on line 337
• Warning: Invalid argument supplied for foreach() in /var/www/html/wordpress/docroot/wp-includes/functions.php on line 3459
• Warning: array_filter() expects parameter 1 to be array, null given in .../customize-snapshots/php/class-customize-snapshot.php on line 314
• Warning: array_keys() expects parameter 1 to be array, null given in .../customize-snapshots/php/class-customize-snapshot.php on line 337