Fix equality and operations (*, +, -) for FlxColor:

- Equality: In cpp a null FlxColor can't exist because there is no null
int, they will be initialized to 0 so if you compare FlxColor == null it
will always be false because 0 == null is false.
- Operations: Because it's possible to make an operation in null members,
fallbacks are needed so it won't happen.
If both members are null the returned color is Black, if only a member
is null the non-null member will be returned, if both are not null the
operation proceeds as expected.