Bug 8296: Follow-up - don't display publisheddate when publisheddatetext is empty

Bug 8296: Follow-up - don't display publisheddate when publisheddatetext is empty

To test:
- make sure you have a subscription where only some fields have a published date (text) set
- check the subscription detail page, issues tab
- check the serial collection page
- both pages should show an empty field, when published date (text) has not been set

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>