fix: prevent a crash if there is a unique key constraint with a nil field.

[POABOB]

Issue #3766.

goctl model mysql ddl --src user_base.sql --dir .

# cat user_base.sql 
CREATE TABLE `t_user_base` (
  `f_id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
  `f_uname` VARCHAR(16),
  `f_phone` VARCHAR(20),
  `f_email` VARCHAR(32),
  `f_stop_time` DATETIME NOT NULL,
  `f_create_time` DATETIME NOT NULL,
  PRIMARY KEY (`f_id`),
  UNIQUE KEY (`f_phone_number`) // error field
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3;

There is a case where the unique key 'f_phone_number' does not match the 'f_phone' field, which can cause a crash without displaying an error message.

And I found that there is a local variable that is not being used, but it will be processed in Parse().
Do I need to mark it as a comment?

[codecov]

Codecov Report

Merging #3770 (e4179e8) into master (22c98be) will decrease coverage by 0.03%.
The diff coverage is n/a.

Additional details and impacted files

see 3 files with indirect coverage changes

[POABOB]

I accidentally take @zzZZzzz888's work.
Could I mark normalIndex as a comment, and submit a PR after he/she done it?

[kesonan]

LGTM