Expression castRealAsTime does not respect sql mode.

[mengxin9014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

 create table t(d1 double, f float);
 insert into t values(0, 0);
 set session sql_mode = "NO_ZERO_IN_DATE,NO_ZERO_DATE";
 select cast(d1 as datetime), cast(f as datetime) from t;

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

+----------------------+---------------------+
| cast(d1 as datetime) | cast(f as datetime) |
+----------------------+---------------------+
| NULL                 | NULL                |
+----------------------+---------------------+

3. What did you see instead (Required)

+----------------------+---------------------+
| cast(d1 as datetime) | cast(f as datetime) |
+----------------------+---------------------+
| 0000-00-00 00:00:00  | 0000-00-00 00:00:00 |
+----------------------+---------------------+

4. What is your TiDB version? (Required)

nightly

Other expressions like castDecimalAsTime, etc. may have the same problem.

[bb7133]

This is about the behavior of cast, reassign it to 'sig-execution'.