input: Write a function to multiply two given matrices and return the resulting matrix. If the matrices are not compatible for multiplication, return None.
  def matrix_multiplication(A, B):
      if len(A[0]) != len(B):
          return None
      rows_A = len(A)
      cols_A = len(A[0])
      rows_B = len(B)
      cols_B = len(B[0])
      result = [[0 for i in range(cols_B)] for j in range(rows_A)]
      for i in range(rows_A):
          for j in range(cols_B):
              for k in range(cols_A):
                  result[i][j] += A[i][k] * B[k][j]
      return result
  assert matrix_multiplication([[1, 2], [3, 4]], [[5, 6], [7, 8]]) == [[19, 22], [43, 50]]
  assert matrix_multiplication([[1, 2]], [[3], [4]]) == [[11]]
  assert matrix_multiplication([[1, 2]], [[3, 4], [5, 6]]) == None
  assert matrix_multiplication([], []) == None
output: def matrix_multiplication(A, B):
      if len(A[0]) != len(B):
          return None
      rows_A = len(A)
      cols_A = len(A[0])
      rows_B = len(B)
      cols_B = len(B[0])
      result = [[0 for i in range(cols_B)] for j in range(rows_A)]
      for i in range(rows_A):
          for j in range(cols_B):
              for k in range(cols_A):
                  result[i][j] += A[i][k] * B[k][j]
      return result