Fix PHP warning in CRM_Core_BAO_CustomOption

Just an undefined array, but one that filled up my watchdog logs, so I hunted it down!
```
Warning: in_array() expects parameter 2 to be array, null given in CRM_Core_BAO_CustomOption::getOptionListSelector() (line 147 of /home/jon/local/civicrm-buildkit/build/dmaster/sites/all/modules/civicrm/CRM/Core/BAO/CustomOption.php)
```

This message fires when you have

• A custom checkbox field;
• No default value is set for the field;
• You click "Edit Multiple Choice Options" for that field.

You'll see the PHP warning in your watchdog log once for each checkbox option you have. PR incoming.

PR tester: If you have a stock demo site, this is easy to test by changing the input type of "Most important issue" to "checkbox" then selecting "Edit Multiple Choice Options" for that field.