[multipath] don't exit if pathvec is empty after discovery

[multipath] don't exit if pathvec is empty after discovery

When all path fail or are removed by operator, but the maps are still
present, expected behaviour is for "multipath -l" to work.

IOW outs something like :

xa-s05:~/scm/multipath-tools-0.4.6# multipath/multipath -l
149455400000000000000000002000000d00600000b000000 ()
[size=67 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [enabled]
 \_ 10:0:0:1 sdd 8:48 [active]

149455400000000000000000002000000d20600000b000000 ()
[size=33 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [enabled]
 \_ 10:0:0:2 sde 8:64 [active]

xa-s05:~/scm/multipath-tools-0.4.6# iscsiadm -m node --logout --record=0203b2
xa-s05:~/scm/multipath-tools-0.4.6# multipath/multipath -l
149455400000000000000000002000000d00600000b000000 ()
[size=67 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [enabled]
 \_ #:#:#:#   8:48 [active]

149455400000000000000000002000000d20600000b000000 ()
[size=33 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [enabled]
 \_ #:#:#:#   8:64 [active]