Fill options value if /proc/cmdline is empty

In case installer have empty /proc/cmdline set options value to "empty"
Otherwise it would fail. bsc#908185

Signed-off-by: Dinar Valeev <dvaleev@suse.com>
this looks wrong. default value should be `""` ideally code should look like
```
options = SCR.Read(path(".proc.cmdline")) || ""
```