Refactor `PropertyMapper` so that it no longer calls adapter or
predicate methods by default when the source value is `null`. This
effectively makes all default calls the same as using
`alwaysWhenNotNull` in the previous generation of the code.
For the limited times when you do need to deal with `null` values, the
new `always()` method can be used.
For example,
map.from(source::method).to(destination::method);
Will not call `destination.method(...)` if `source.method()` returns
`null`.
Where as:
map.from(source::method).always().to(destination::method);
Will call `destination.method(null)` if `source.method()` returns
`null`.
This update provides clearer semantics for the API and allows for better
JSpecify nullability annotations. It has also simplified much of our
existing property mapper code.
Closes gh-47024
Co-authored-by: Moritz Halbritter <moritz.halbritter@broadcom.com>
Stéphane Nicoll
Moritz Halbritter
Andy Wilkinson
Phillip Webb