docs: @Nonnull
      public static <E> Set<E> union(@Nullable final Set<E> pColl1, @Nullable final Set<E> pColl2)
      {
          final Set<E> result = new HashSet<>();
          if (pColl1 != null) {
              result.addAll(pColl1);
          }
          if (pColl2 != null) {
              result.addAll(pColl2);
          }
          return Collections.unmodifiableSet(result);
      }