public static <T> Comparator<T> combine(final Comparator<T> comparator1, final Comparator<T> comparator2) {
        if (comparator1 == null) {
            return comparator2;
        } else if (comparator2 == null) {
            return comparator1;
        } else {
            return comparator1.thenComparing(comparator2);
        }
    }
Combine two comparators in a null safe way.
If one arg is null, the other arg is returned.
If both are null, null is returned.
If both args are non-null it is equivalent to:
<pre>{@code
comparator1.thenComparing(comparator2);
}</pre>
stroom-util-shared/src/main/java/stroom/util/shared/CompareUtil.java