GTNPORTAL-3370 - Fix up for endPublicationTime as empty

When the endPublicationTime is cleared on the UI, it's set as -1 on the permanent storage. So, we need to check for this case. Previously, pages without endPublicationTime and privateTillPublicationDate set to true would have the access blocked, as endPublicationTime would mistakenly be interpreted as being on the past.