properties: fix bad creation and modification date displayed
As explained in the issue #1549, according to pdf spec, creation date and
modification date are optional (for pdf documents but others document type
don't use theses dates).
I expect a file not to be modified exactly at
31 Dec 1969 23:59:59 UTC, even
if someone has modified the information in the file for fun. So each creation
and modification date equal to
-1 is considered as
NULL for the pointers to
the other attributes and None is displayed.
The if statements could be used in the future when using
GDateDime instead of
GTime (a pointer could help, or optional type).
N.B. This is my very first contribution, perhaps I made some mistakes: please tell me!