Nullable Timestamp Column Fix

[GuptaManan100]

Description

Running a query like

create table foo (
    id int primary key,
    a varchar(255) null,
    b varchar(255) null default 'foo',
    c timestamp null default current_timestamp()
)

on vitess got rewritten to

create table foo (
    id int primary key,
    a varchar(255),
    b varchar(255) default 'foo',
    c timestamp default current_timestamp()
)

but this is incorrect because by default timestamp column is not nullable. This PR fixes this issue.

Related Issue(s)

• Fixes Incorrect Nullable value in timestamp column #7739

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin