[khromenokroman]

@olofhagsand Hello

Description:
Fixes #647

Problem

validate_leafref() returned OK immediately if the leafref node had no body:

  if ((leafrefbody = xml_body(xt)) == NULL)
      goto ok;

So an empty value was always accepted, even with require-instance true and
no referred instance with an empty value:

  set test object object1 group ""
  validate   <- OK, but there is no group ""

Fix

An empty value is a value like any other (RFC 7950 9.9): it must refer to an
existing instance with the same value.

• Treat a missing body as an empty string, both for the leafref node and for
  the referred instances. An empty leafref is valid only if some referred node
  also has an empty value.
• The LEAFREF_OPTIMIZE binary search builds a lookup key from the value, so
  use the linear search for an empty value. Empty leafrefs are rare, so this
  has no performance impact.
• validate_leafref_err(): print an empty body instead of (null) in the
  instance-required error, i.e. <error-info><group/></error-info>.

Behavior change

Configurations that contain an empty leafref without a matching empty referred
value were accepted before and now fail validation with instance-required.
This is the intended fix, but it may affect existing datastores.

Testing

• New test/test_leafref_empty.sh (separate file to not interfere with Fix leafref binary search accepting entries excluded by xpath predicates #700):
  empty leafref to a leaf-list (the scenario from An empty value passes leafref validation #647), to a list key (binary
  search path) and to a non-key leaf must fail; an empty leafref to an existing
  empty value must pass.

• Verified the same YANG and data with a standalone validation program
  (xml_yang_validate_all_top):

  case	master	this PR
  empty ref to leaf-list	pass	fail
  empty ref to list key (binary search)	pass	fail
  empty ref to non-key leaf	pass	fail
  empty ref, referred node has empty value	pass	pass
  non-empty valid / invalid ref	pass/fail	pass/fail

• Performance unchanged: 20000 list entries + 20000 leafrefs validate in the
  same time as on master.

• Merges cleanly with Fix leafref binary search accepting entries excluded by xpath predicates #700.