Fix validation of SBD devices when none is specified

https://bugzilla.novell.com/show_bug.cgi?id=888060
Still don't understand :-/  Seems to me there are a lot more than two cases:

* 1 node cluster
    * SBD device not populated: `[""]`
    * SBD device populated: `["/dev/foo"]`
* 2 node cluster
    * no SBD devices populated: `["", ""]`
    * one SBD device populated: `["/dev/foo", ""]` or `["", "/dev/foo"]`
    * both SBD devices populated: `["/dev/foo", "/dev/bar"]`
* 3-node cluster
    * no SBD devices populated: `["", "", ""]`
    * one SBD device populated: `["/dev/foo", "", ""]` or `["", "/dev/foo", ""]` or `["", "", "/dev/foo"]`
    * two SBD devices populated: `["/dev/foo", "/dev/bar", ""]` or `["/dev/foo", "", "/dev/bar"]` or `["", "/dev/foo", "/dev/bar"]`
    * all three SBD devices populated: `["/dev/foo", "/dev/bar", "/dev/bar/baz]`
* 4 node cluster
    * ...
* ...

Also I'm not sure what you mean by "empty array with an empty string" - isn't that an oxymoron?  `[""]` is not an empty array, so maybe I'm misunderstanding what you mean?  

Thanks for your patience with me on this!!