input: Write a function to generate a two-dimensional array filled with zeros with given dimensions (row and column). If the given dimensions are invalid, return None.
  
  def generate_zeros_array(row, col):
      if row <= 0 or col <= 0:
          return None
      return [[0] * col for _ in range(row)]
  
  # Example
  assert generate_zeros_array(3, 3) == [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
  assert generate_zeros_array(-1, 3) == None
  assert generate_zeros_array(3, -1) == None
  assert generate_zeros_array(0, 0) == None
output: def generate_zeros_array(row, col):
      if row <= 0 or col <= 0:
          return None
      return [[0] * col for _ in range(row)]