plug-ins: Do not write to Iptc.Application2.DateCreated if no date was set.

The metadata-editor allowed Iptc tags to be set if an empty string was
used contrary to Xmp tags. In the case of DateCreated this cause an
invalid date "0-00-00" to be written.

We added a check to only write text Iptc metadata if the value is
not empty.

(cherry picked from commit 0a902456)