Fix distro check

Merged Yoshiharu Hirose requested to merge yh1224/cerbero:yh1224-main-patch-80844 into main

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

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