input: Write a function to generate a matrix with given rows and columns filled with zeros.
  assert generate_matrix(2, 3) == [[0, 0, 0], [0, 0, 0]]
  assert generate_matrix(4, 4) == [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
  assert generate_matrix(1, 5) == [[0, 0, 0, 0, 0]]
output: def generate_matrix(rows, columns):
      return [[0 for _ in range(columns)] for _ in range(rows)]