Customize: Fix PHP notice when calling `WP_Customize_Control::json()` inside...

Customize: Fix PHP notice when calling `WP_Customize_Control::json()` inside `content_template()` method.

A `temp` control is instantiated when `WP_Customize_Manager:: render_control_templates()` is called. This control needs to explicitly specify an empty settings array to avoid trying to use a `temp` setting which won't exist.

See #35926.
See #29572.

Built from https://develop.svn.wordpress.org/trunk@36776


git-svn-id: https://core.svn.wordpress.org/trunk@36743 1a063a9b-81f0-0310-95a4-ce76da25c4cd