Don't use timestamps with all zero

Although all zero (0000:00:00) is valid time, in most cases it means
that value is not present. According to http://www.exiv2.org/Exif2-2.PDF
in such case those values should be omitted, however
some cameras set them to 0 anyway. With this commit such timestamps
will be treated as they were empty.

Change-Id: I9c762b1fa04ea6bf9c0fba9e2459a20430c71c90