LLMS post date properties (and WP 5.3): make them nullable and account for date_floating property

[eri-trabiccolo]

In WP 5.3 there will be some changes that will impact on the wp posts date properties.
They will impact the rest-api as well as you can see in the following two commits:
(null dates) WordPress/WordPress@b15c0d4#diff-2bdd57a49dbaa7ba0108f91132d37216
(floating date)
WordPress/WordPress@ef86b03#diff-2bdd57a49dbaa7ba0108f91132d37216

Interesting the possibility of specifying the property type as an array:
WordPress/WordPress@b15c0d4#diff-337a27824fef93e1a776fe4a26fff320R1904

which anyways requires the wp core functions rest_sanitize_value_from_schema and rest_validate_value_from_schema
WordPress/WordPress@b15c0d4#diff-04b011c39a32e2b17ffb640e03cd3d01R1211
WordPress/WordPress@b15c0d4#diff-04b011c39a32e2b17ffb640e03cd3d01R1390

I think we can - but didn't test it - create our own wrappers for these two functions (which actually means make also wrappers for rest_sanitize_request_arg and rest_validate_request_arg) to handle the backward compatibility with older wp versions and then make these:
https://github.com/gocodebox/lifterlms-rest/blob/1.0.0-beta.8/includes/abstracts/class-llms-rest-posts-controller.php#L1008-L1017
something like:

				'date_created'     => array(
					'description' => __( 'Creation date. Format: Y-m-d H:i:s', 'lifterlms' ),
					'type'        => array( 'string', 'null' ),
					'context'     => array( 'view', 'edit' ),
					'arg_options' => array(
						'sanitize_callback' => 'llms_rest_sanitize_request_arg',
						'validate_callback' => 'llms_rest_validate_request_arg'
					),
				),
				'date_created_gmt' => array(
					'description' => __( 'Creation date (in GMT). Format: Y-m-d H:i:s', 'lifterlms' ),
					'type'        => array( 'string', 'null' ),
					'context'     => array( 'view', 'edit' ),
					'arg_options' => array(
						'sanitize_callback' => 'llms_rest_sanitize_request_arg',
						'validate_callback' => 'llms_rest_validate_request_arg'
					),
				),

(and of course we can create our wrapper for has_param() too).