lxd/storage/utils: Removes default empty "size" property for dir volumes
lxd/storage/utils: Removes default empty "size" property for dir volumes

 - VolumeFillDefault now does not populate an empty string for the "size" property for new volumes.
 - This is aligned with the other storage types that support the size property.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

hallyn/lxd,Skarlso/lxd,atxwebs/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,mjeanson/lxd,mjeanson/lxd,Skarlso/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,Skarlso/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,mjeanson/lxd,Skarlso/lxd