fix: didn't pass non-exist value when call Node.Interface()

[AsterDY]

Issue

After Unset() some nodes for its parent, call parent.Interface() will return errors like unsupported type

Reason

That's because since # , sonic/ast didn't delete node memory after Unset() meanwhile Interface() logic counted the actually non-exist node

[codecov-commenter]

Codecov Report

Merging #544 (62d514d) into main (de254c3) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main     #544   +/-   ##
=======================================
  Coverage   78.30%   78.30%           
=======================================
  Files          69       69           
  Lines       10686    10686           
=======================================
  Hits         8368     8368           
  Misses       1955     1955           
  Partials      363      363           

Files	Coverage Δ
ast/iterator.go	70.11% <100.00%> (+1.44%)	⬆️
ast/node.go	82.80% <100.00%> (-0.08%)	⬇️

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