input: Write a function to filter out all negative numbers from a given matrix and return the modified matrix where all negative numbers are replaced with 0. 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[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([[1, -2, 3], [-4, 5, 6]]) == [[1, 0, 3], [0, 5, 6]]
  assert filter_out_negative([[1, 2, 3]]) == [[1, 2, 3]]
  assert filter_out_negative([[-1], [2], [-3]]) == [[0], [2], [0]]
  assert filter_out_negative([]) == []
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