handle empty migration list case for wide listing

[reedstrm]

Hit this traceback recently:

Traceback (most recent call last):
  File "/home/reedstrm/src/rewrite/bin/dbmigrator", line 11, in <module>
    load_entry_point('db-migrator', 'console_scripts', 'dbmigrator')()
  File "/home/reedstrm/src/rewrite/db-migrator/dbmigrator/cli.py", line 116, in main
    return args['cmmd'](**args)
  File "/home/reedstrm/src/rewrite/db-migrator/dbmigrator/utils.py", line 155, in wrapper
    return func(cursor, *args, **kwargs)
  File "/home/reedstrm/src/rewrite/db-migrator/dbmigrator/commands/list.py", line 29, in cli_command
    name_width = max([len(name) for _, name, _ in migrations])
ValueError: max() arg is an empty sequence

This code solves it.

[coveralls]

Coverage decreased (-0.4%) to 97.366% when pulling e981281 on Connexions:wide-empty-list into 7a8e0e2 on karenc:master.