[3.32] GitgHistory.RefRow: Don't assume that every GgitSignature has a time
The libostree and libreswan git repositories include counterexamples.
Signed-off-by: Simon McVittie smcv@debian.org Fixes: #73 (closed) Bug-Debian: https://bugs.debian.org/935136
(cherry picked from commit b49eca0b)