feat: Add support for condition single operator (no value)

[nlemoine]

Hey @stevep,

ACF is now providing some single operator (e.g. no mandatory value):

'conditional_logic' => array(
    array(
        array(
            'field' => 'field_66db320aef59b',
            'operator' => '==empty',
        ),
    ),
),

This PR adds support for them.

Since the LocationBuilder is extending the Conditionalbuilder, I had to make the LocationBuilder::createCondition compatible, hence the exception throw.

Let me know if this needs any changes.