input: Write a Python function called `pluck_node` that, given an array representing a branch of a tree with non-negative integer nodes, selects and returns one of the nodes. The selected node should have the smallest even value, and if there are multiple nodes with the same smallest even value, the node with the smallest index should be chosen. The result should be returned as a list containing the smallest value and its index. If no even values exist or the array is empty, return an empty list.
output: def pluck_node(arr):
      smallest_even = None
      index = -1
      for i, value in enumerate(arr):
          if value % 2 == 0:
              if smallest_even is None or value < smallest_even:
                  smallest_even = value
                  index = i
      return [smallest_even, index] if smallest_even is not None else []