input: Write a function to filter out all negative numbers from the given matrix. If the given matrix is empty then return an empty matrix.
  
  
  def filter_out_negative(matrix):
      if not matrix:
          return []
      for i in range(len(matrix)):
          for j in range(len(matrix[i])):
              if matrix[i][j] < 0:
                  matrix[i][j] = 0
      return matrix
  
  assert filter_out_negative([[-1, 2, -3], [-4, 5, 6], [-7, 8, 9]]) == [[0, 2, 0], [0, 5, 6], [0, 8, 9]]
  assert filter_out_negative([]) == []
  assert filter_out_negative([[1, 2], [3, 4]]) == [[1, 2], [3, 4]]
  assert filter_out_negative([[-1, -2, -3], [-4, -5, -6]]) == [[0, 0, 0], [0, 0, 0]]
  
output: def filter_out_negative(matrix):
      if not matrix:
          return []
      for i in range(len(matrix)):
          for j in range(len(matrix[i])):
              if matrix[i][j] < 0:
                  matrix[i][j] = 0
      return matrix