BUG: passing null value to prepare_date_items() leads to deprecation warning in strtotime()

[barryceelen]

Describe the bug

The $post_date variable passed to Post->prepare_date_items() in Post->prepare_document() can be null. Post->prepare_date_items() uses that value in a strtotime() call. Passing null to strtotime() results in a PHP deprecation warning.

...Deprecated: strtotime(): Passing null to parameter #1 ($datetime) of type string is deprecated in...

Steps to Reproduce

Indexing a post with $post->post_date set to 0000-00-00 00:00:00 or an otherwise invalid $post->post_date value results in the $post_date variable being set to null:

https://github.com/10up/ElasticPress/blob/develop/includes/classes/Indexable/Post/Post.php#L463

Code of Conduct

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[felipeelia]

Thanks @barryceelen! Any chance you can open a PR with the fix?