Fix 'mel list --sort lastmicro' when no micro
Fix 'mel list --sort lastmicro' when no micro

Moles that have no microscope images should be sortable and should be
consider 'less than' or 'ordered before' moles with microscope images.

The previous behaviour of considering moles without microscope images as
'None' would result in a TypeError as 'str()' and 'None' are not
comparable.

Test Plan:
$ ./meta/run_tests.sh

.. Change to a data set directory ..
$ mkdir mole_without_micro
$ touch mole_without_micro/file
$ mel list --sort lastmicro
.. No exception raised ..

aevri/mel,aevri/mel