Add validation for the custom publish date in studio.

[padms]

Describe the bug
News article published date is null when custom publication date is set to true and no publish date time is given. On publish newsletter fails, no published date appears on the article and it does not appear or newsroom or search.

To Reproduce
Steps to reproduce the behavior:

1. Create a news article, by selecting the custom publish date to true, but do not select the publish time.
2. Publish the news article. News article publishes without validation.

Expected behavior
Validate the custom publish date when set to true.