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