Fix: Fail gracefully if there are no (valid) commits

[igordertigor]

This error occurs if there are no valid commits since the last tag:

Traceback (most recent call last):
   File "/home/ingo/code/semv/.tox/cram/bin/semv", line 8, in <module>
     sys.exit(main())
   File "/home/ingo/code/semv/.tox/cram/lib/python3.8/site-packages/semv/main.py", line 8, in main
     print(commands.version_string(), end='')
   File "/home/ingo/code/semv/.tox/cram/lib/python3.8/site-packages/semv/commands.py", line 36, in version_string
     inc = vi.get_version_increment(commits)
   File "/home/ingo/code/semv/.tox/cram/lib/python3.8/site-packages/semv/increment.py", line 30, in get_version_increment
     return min(
 ValueError: min() arg is an empty sequence