orm: fix option field with default null value

[kbkpbot]

This PR fix PR #24222

For struct option fields with default values, a null in database field should update the struct's field to none, not it's default value.

@[table: 'sys_users']
struct UserPart {
	id         int @[primary; serial]
	name       string
	created_at time.Time @[sql_type: 'TIMESTAMP']
	updated_at time.Time @[sql_type: 'TIMESTAMP']
	option_i8	?i8 = 13		// option with default test
	option_string ?string = 'this is not none'
}

for example, if database field option_i8, option_string are null, then these fields should set to none, not their default values.

[huly-for-github]

Connected to Huly®: V_0.6-22610