[lastpostmodified] Need to prevent override from returning a zero'd timestamp

[trepmal]

https://github.com/Automattic/vip-go-mu-plugins/blob/89b258d45f621dec7356a4867dbf2a6ed025e257/performance/lastpostmodified.php#L38

In one instance, this returned a 0000-00-00 00:00:00 timestamp (unknown cause). We should prevent that from happening, either here or when the value is saved.

When the timestamp is zero'd, WP will end up returning a 304 for feeds to Varnish’s request for content, even though there are no conditional headers present in the request. This is an RFC violation and results in the error 304 response but not conditional fetch in Varnish, resulting in a 503 being sent to the client even though the site is up and running.

[simonwheatley]

Noted in JIRA as GH-3339

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