Update Post Object, Page Link, User Fields and Select Fields to include allow_null

[mattmoore83]

Found when using a select that you can't unselect once set, recommend including allow_null by default and use the already passed required parameter to reverse logic so 'allow_null' => !$required.

[keefyhub]

Can we include this on taxonomy fields too (not shown as an option when creating in the CMS but appears to be available in PHP)