input: Write a function to perform matrix multiplication on two given matrices. If the number of columns in the first matrix is not equal to the number of rows in the second matrix, return None.
  assert matrix_multiplication([[1, 2], [3, 4]], [[5, 6], [7, 8]])==[[19, 22], [43, 50]]
  assert matrix_multiplication([[1, 2, 3], [4, 5, 6]], [[7, 8], [9, 10], [11, 12]])==[[58, 64], [139, 154]]
  assert matrix_multiplication([[1, 2], [3, 4]], [[1, 2, 3], [4, 5, 6]])==None
  assert matrix_multiplication([], [])==None
  assert matrix_multiplication([[1]], [[2]])==[[2]]
output: def matrix_multiplication(A, B):
      if not A or not B or len(A[0]) != len(B):
          return None
      result = [[0 for _ in range(len(B[0]))] for _ in range(len(A))]
      for i in range(len(A)):
          for j in range(len(B[0])):
              for k in range(len(B)):
                  result[i][j] += A[i][k] * B[k][j]
      return result