db: Empty dates should be returned as null by typecast

[SaschaKoth]

If a date value is an empty string (''), the typecast should return null because e.g. pgsql dismiss empty strings. Swapping the if statements does this.

Please review!

Thanks
Sascha

[coveralls]

Coverage remained the same when pulling 939c46d on SaschaKoth:master into 64862f9 on yiisoft:master.

[cebe]

I remember we rejected similar request as it is magic and not necessarily wanted.
If you have empty string set to ActiveRecord for example you should use default value filter in validation rules to set it to null explicitly.

[qiangxue]

Related: yiisoft/yii#2056

[YanAlex]

2022 is NOW!

[['time_start', 'time_end'], 'default', 'value' => null],
Still write to MYSQL 00:00:00 instead of NULL.

FIX THIS ALREADY!!!