Php SilverStripe DropdownField默认值设置为空

有人能帮我在SilverStripe 3中的

下拉字段中设置默认为空吗

目前我的代码如下：

DropdownField::create(
    'HowHear',
    'How did you hear of this job?',
    array(
        'Indeed' => 'Indeed',
        'Caterer' => 'Caterer',
        'Gumtree' => 'Gumtree',
        'Word of mouth' => 'Word of mouth',
        'Other' => 'Other',
    )
)->addExtraClass('full-width'),

在SilverStripe 3和4中，DropdownField有一个函数可以设置默认的空字符串：

DropdownField::create(
    'HowHear',
    'How did you hear of this job?',
    [
        'Indeed' => 'Indeed',
        'Caterer' => 'Caterer',
        'Gumtree' => 'Gumtree',
        'Word of mouth' => 'Word of mouth',
        'Other' => 'Other',
    ]
)->addExtraClass('full-width')->setEmptyString('')

在SilverStripe 3和4中，DropdownField有一个函数可以设置默认的空字符串：

DropdownField::create(
    'HowHear',
    'How did you hear of this job?',
    [
        'Indeed' => 'Indeed',
        'Caterer' => 'Caterer',
        'Gumtree' => 'Gumtree',
        'Word of mouth' => 'Word of mouth',
        'Other' => 'Other',
    ]
)->addExtraClass('full-width')->setEmptyString('')