Write a function that finds the sum of even elements on the main diagonal of a square matrix (namely, even elements, not elements in even positions!). If there are no even elements, then output 0. Use the numpy library.

import numpy as np

def diagonal_sum(matrix):
    diagonal = np.diagonal(matrix)
    return sum([x for x in diagonal if x%2==0])

matrix = np.array([[1,2,3], [4,5,6], [7,8,9]])
print(diagonal_sum(matrix))