BIND 10 #1065: support empty node in DatabaseZoneHandle::find().

[BIND 10 Development]

#1065: support empty node in DatabaseZoneHandle::find().
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  major         |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 See commit 76e684704466abf66792b79582c716b83ff4da5b of trac817a for
 prototype implementation.

 (but the actual implementation doesn't have to stick to the way that
 the prototype does)

 This task depends on #1060, #1061, #1062 (but can be done in parallel
 with other find() support tasks).

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Ticket URL: <http://bind10.isc.org/ticket/1065>
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BIND 10 Development