Fix distro check
When distro_version is blank string, the error has occured below.
$ ./cerbero-uninstalled bootstrap
Traceback (most recent call last):
File "<string>", line 19, in <module>
File "/tmp/cerbero/cerbero/main.py", line 19, in <module>
from cerbero import hacks
File "/tmp/cerbero/cerbero/hacks.py", line 99, in <module>
from cerbero.utils.shell import new_call as shell_call
File "/tmp/cerbero/cerbero/utils/shell.py", line 51, in <module>
PLATFORM = system_info()[0]
File "/tmp/cerbero/cerbero/utils/__init__.py", line 250, in system_info
elif distro_version in ['focal'] or (distro_version[0] == 'u' and d[1].startswith('20.')):
IndexError: string index out of range