Don't copy sexpr node value that is an empty string

Don't copy sexpr node value that is an empty string

Xen4.1 initializes some unspecified sexpr config items to an empty
string, unlike previous Xen versions that would leave the item unset.
E.g. the kernel item for an HVM guest (non-direct kernel boot):

Xen4.0 and earlier
  ...
  (image
    (hvm
      (kernel )
  ...

Xen4.1
  ...
  (image
    (hvm
      (kernel '')
  ...

The empty string for kernel causes some grief in subsequent parsing
where existence of specified kernel is checked, e.g.

  if (!def->os.kernel)
  ...

This patch solves the problem in sexpr_node_copy() by not copying
a node containing an empty string.