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