Dpkg::Changelog: Cope gracefully with a missing trailer timestamp

The get_timepiece methods might return an undef value if there was no
timestamp in the trailer. Do not call the Time::Piece epoch method in
that case, as that causes a perl error.

Regression introduced in commit ea22158e.

Closes: #843248


Based-on-patch-by: Ian Jackson <ijackson@chiark.greenend.org.uk>