Cannot change expire date on an existing contentlet to 'Never'

[swicken-dotcms]

Describe the bug You cannot change the expire date on an existing contentlet to 'never'. It throws the error Unable to convert string to date +0000

This issue makes it so you cannot manually unpublish content that has been published with an expire date.

Related ticket: https://dotcms.zendesk.com/agent/tickets/108062

To Reproduce

1. Create a content type with Publish and Expire fields
2. Create a contentlet of that content type, set the expire time to some time in the future
3. Save, then re-open the contentlet
4. Click the 'never' option and attempt to save the contentlet
5. You will see the following error:

Expected behavior You should be able to remove expiry dates from contentlets with the 'never' option