expression: fix issue that TimeIsNull is not compatible with MySQL

[qw4990]

What problem does this PR solve?

Fix #9763.

TimeIsNull in TiDB doesn't check if these DATETIME columns are declared as NOT NULL.
But In MySQL:

For DATE and DATETIME columns that are declared as NOT NULL, 
you can find the special date '0000-00-00' by using a statement like this:
"SELECT * FROM tbl_name WHERE date_column IS NULL"

This PR needs to be merged after pingcap/tipb#121.

For TiKV, since

1. TimeIsNull is not pushed down to TiKV, and
2. there is some problem(can't upgrade dependency tipb in tikv tikv/tikv#4864) to let tipb can't be upgraded in TiKV,

we don't fix TimeIsNull on TiKV.

What is changed and how it works?

Check if these DATETIME columns are declared as NOT NULL.

Check List

Tests

• Unit test

[qw4990]

We have to merge pingcap/tipb#121 firstly to let idc-jenkins-ci-tidb/build pass.

[winoros]

What's result of the following case:

The column can contains NULL.
But we filter NULL out at some places.
Like the SQL: select * from (select * from t where date_col is not null) t where date_col is null;

Why i mention this case is because that we'll maintain the NOT NULL during planning phase. Currently the flag is false when building the TIME IS NULL. It or another field will be true in the future.

[qw4990]

Wait for #10790 and use it to solve this problem instead of introducing a new function signature.