Plan Cache get wrong result when reuse the PointGet plan with null parameter

[Reminiscent]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

func (s *testSuiteP2) TestPointGetPreparedPlan(c *C) {
	tk1 := testkit.NewTestKit(c, s.store)
	tk1.MustExec("drop database if exists ps_text")
	defer tk1.MustExec("drop database if exists ps_text")
	tk1.MustExec("create database ps_text")
	tk1.MustExec("use ps_text")

	tk1.MustExec(`create table t (a int, b int, c int,
			primary key k_a(a),
			unique key k_b(b))`)
	tk1.MustExec("insert into t values (1, 1, 1)")
	tk1.MustExec("insert into t values (2, 2, 2)")
	tk1.MustExec("insert into t values (3, 3, 3)")

	pspk1Id, _, _, err := tk1.Se.PrepareStmt("select * from t where a = ?")
	c.Assert(err, IsNil)
	tk1.Se.GetSessionVars().PreparedStmts[pspk1Id].(*plannercore.CachedPrepareStmt).PreparedAst.UseCache = false
	pspk2Id, _, _, err := tk1.Se.PrepareStmt("select * from t where ? = a ")
	c.Assert(err, IsNil)
	tk1.Se.GetSessionVars().PreparedStmts[pspk2Id].(*plannercore.CachedPrepareStmt).PreparedAst.UseCache = false

	ctx := context.Background()
	// first time plan generated
	rs, err := tk1.Se.ExecutePreparedStmt(ctx, pspk1Id, []types.Datum{types.NewDatum(0)})
	c.Assert(err, IsNil)
	tk1.ResultSetToResult(rs, Commentf("%v", rs)).Check(nil)

	// using the generated plan but with different params
	rs, err = tk1.Se.ExecutePreparedStmt(ctx, pspk1Id, []types.Datum{types.NewDatum(nil)})
}

2. What did you expect to see? (Required)

Execute success.

3. What did you see instead (Required)

Get Panic: interface conversion: interface {} is *executor.TableDualExec, not *executor.PointGetExecutor (PC=0x13998D8)

4. What is your TiDB version? (Required)

Latest master