PHPAPI-16 : PHP SDK does not works if in 'Linked select' custom field, child option is not available for the specified value.

BACKGROUND: While updating custom field value of a linked select field without the child option, the following error gets generated : 
Fatal error: Call to a member function getValue() on a non-object in kyCustomFieldLinkedSelect.php on line 57

Signed-off-by: Saloni Dhall <saloni.dhall@kayako.com>