Don't fail if the OS has no version

[City-busz]

This fixes the following error:

Traceback (most recent call last):
File "/usr/bin/warehouse", line 48, in <module>
sys.exit(main.main(VERSION))
~~~~~~~~~^^^^^^^^^
File "/usr/share/warehouse/src/main.py", line 284, in main
app = WarehouseApplication()
File "/usr/share/warehouse/src/main.py", line 71, in __init__
os_string = GLib.get_os_info("NAME") + " " + GLib.get_os_info("VERSION")
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
TypeError: can only concatenate str (not "NoneType") to str