[5.5] fix updating nullable dates

[themsaid]

Currently if a nullable timestamp field had a value of null, when you update it ELoquent will attempt to cast the original value to Carbon instance, and since the value is null it's going to error.

Here's where we compare original and current value to build the dirty array:

https://github.com/laravel/framework/blob/master/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php#L1007-L1009

This change will check if the given value is null and just return it as is without attempting to cast it as a carbon instance.