apport-gtk crashes on reports without timestamp

[Bart Goeman]

$ /usr/share/apport/apport-gtk
Traceback (most recent call last):
  File "/usr/share/apport/apport-gtk", line 598, in <module>
    app.run_argv()
  File "/usr/lib/python3/dist-packages/apport/ui.py", line 694, in run_argv
    return self.run_crashes()
  File "/usr/lib/python3/dist-packages/apport/ui.py", line 245, in run_crashes
    logind_session[1] > self.report.get_timestamp():
TypeError: '>' not supported between instances of 'float' and 'NoneType'

/usr/lib/python3/dist-packages/apport/report.py:

    def get_timestamp(self):
        '''Get timestamp (seconds since epoch) from Date field

        Return None if it is not present.
        '''

so get_timestamp can return None, but run_crashes does not cover this

$ dpkg -s python3-apport
Package: python3-apport
Status: install ok installed
Priority: optional
Section: python
Installed-Size: 551
Maintainer: Martin Pitt <email address hidden>
Architecture: all
Source: apport
Version: 2.20.7-0ubuntu3.7

$ lsb_release -rd
Description: Ubuntu 17.10
Release: 17.10

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.