Add a nullsafe mode

[roookeee]

As much as datus is about explicitness: traversing nested objects which can be null is quite cumbersome as it stands.

We have to find a way to avoid something like this:

.from(Order::getCustomer)
    .given(Objects::nonNull, Customer::getBillingAddress).orElseNull()
    .given(Objects::nonNull, Address::getCity).orElseNull()
    .into(OrderDTO::setBillingCity)

I personally would like to veto any variant that would make every mapping step of a mapping definition nullsafe - it's too implicit.

[roookeee]

Maybe add a nullsafe() function that turns all map calls etc. into implicit null safe variants. This looks like the most elegant solution, but we lose explicitness and pay a slight performance penalty for any subsequent intra-step maps that don't need to handle null.

EDIT: Thinking about it: when nullsafe() is used all subsequent steps have to be null safe too so this argument is void

[roookeee]

Add a mapNullsafe() (bad name, just an example) ? It's quite verbose but has no other drawbacks compared to the aforementioned variant.

On the other hand: once you have done one mapNullSafe() all the following steps have to be nullsafe too which makes this approach less appealing

[roookeee]

A working implementaion is in PR #22. Instead of adding a boolean flag for the implicit null checking I opted for an extendable enum to not break the public APIs as easily in the future

[roookeee]

Will probably do a 1.3.0 release this weekend that includes this feature as it's quite useful

[roookeee]

This feature will be released tomorrow after a local review

[roookeee]

Released