qemu: Don't update physical storage size of empty drives

qemu: Don't update physical storage size of empty drives

Previously the code called virStorageSourceUpdateBlockPhysicalSize which
did not do anything on empty drives since it worked only on block
devices. After the refactor in c5f6151390 it's called for all devices
and thus attempts to deref the NULL path of empty drives.

Add a check that skips the update of the physical size if the storage
source is empty.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1420718