Fix the null field issue responses CONCAT(NULL, ...)

[MrSwitch]

For one to many relationships puts the fields from the many table into a CONCAT, i.e...

{ 
    one_id,
    many: {
        many_id, 
        could_be_null_field
    }
}

// Results in...
// ... GROUP_CONCAT(CONCAT(many_id, could_be_null_field))...

Which if could_be_null_field is null the whole row will be excluded.

CONCAT() returns NULL if any argument is NULL.

https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_concat