Allow CustomValueComparators to handle nulls

Currently, before calling a CustomValueComparator impl, Javers checks nulls and assumes that :

null != any non-null value

This assumption simplifies an impl, but from the other hand it makes impossible to write a Comparator which treats nulls as empty values. See https://stackoverflow.com/questions/66497652/javers-comparison-string-collection-and-boolean-with-defined-rules

public interface CustomValueComparator<T> {
    /**
     * Called by JaVers to compare two Values.
     *
     * @param a not null
     * @param b not null
     */
    boolean equals(T a, T b);

New feature Skip the Javers internal null-safety logic and allow passing nulls to CustomValueComparators.

Unfortunately it could break existing users’ code

Test case CaseWithCustomComparatorBlejwas.groovy

[bluebartle]

Thanks @bartoszwalacik - and good point. I’ll make the naming more straightforward as suggested. I’ll get a PR raised presently.

[bartoszwalacik]

Ok @ratskinmahoney, your design looks good, but maybe we can avoid double negations?

public interface CustomValueComparator<T> {
  
  // ...

  default boolean nullSafety() { return true; }
}