Check for empty attributes on SF volume

Check for empty attributes on SF volume

This should not happen, but; we should be more strict
about our dict reading/checking and always make sure
the key item actually comes back and exists.

In the case of volume['attributes'] we were always
assuming this to be present which should be "ok", but
it's better to verify if rather than assuming, so this
patch adds a simple verification step before trying to
fetch on volume['attributes'].

Change-Id: I37a134a735aaf043aed1230d57fe458bb3a60240
Closes-Bug: #1495247