TIMESTAMP logic remains partly non-standard with explicit_defaults_for_timestamp

Note: Even though this is an old known issue not specific to 10.10, I think it becomes much more important now when MDEV-28632 is about to be pushed into 10.10 main; so I'm marking it as 10.10V1 against the standard procedure. Please feel free to adjust both the marking and the priority after deciding what to do about it.

With explicit_defaults_for_timestamp, if a TIMESTAMP column is defined as NOT NULL without a default value, it still partially retains an implicit (only undeclared) logic and gets a current timestamp value if NULL is attempted to be inserted.

Run with --mysqld=--explicit-defaults-for-timestamp=on

CREATE TABLE t (a TIMESTAMP NOT NULL);

SHOW CREATE TABLE t;

INSERT INTO t VALUES (NULL);

SELECT * FROM t;

# Cleanup

DROP TABLE t;

CREATE TABLE t (a TIMESTAMP NOT NULL);

SHOW CREATE TABLE t;

Table	Create Table

t	CREATE TABLE `t` (

  `a` timestamp NOT NULL

) ENGINE=MyISAM DEFAULT CHARSET=latin1

INSERT INTO t VALUES (NULL);

SELECT * FROM t;

a

2022-06-30 00:34:21