storage: mpath: Don't error on target_type=NULL

storage: mpath: Don't error on target_type=NULL

We use device-mapper to enumerate all dm devices, and filter out
the list of multipath devices by checking the target_type string
name. The code however cancels all scanning if we encounter
target_type=NULL

I don't know how to reproduce that situation, but a user was hitting
it in their setup, and inspecting the lvm2/device-mapper code shows
many places where !target_type is explicitly ignored and processing
continues on to the next device. So I think we should do the same

https://bugzilla.redhat.com/show_bug.cgi?id=1069317