Method unmodifiableOrNull

guava/src/com/google/common/collect/Maps.java:3410–3413

(@Nullable Entry<K, ? extends V> entry)

  }

  private static <K extends @Nullable Object, V extends @Nullable Object>
      @Nullable Entry<K, V> unmodifiableOrNull(@Nullable Entry<K, ? extends V> entry) {
    return (entry == null) ? null : unmodifiableEntry(entry);
  }

  @GwtIncompatible // NavigableMap
  private static final class UnmodifiableNavigableMap<