message: Implement ColKey::nullvalue like done for TableKey
  Should not change anything. But makes an assertion about the value.
diff: @@ -28,6 +28,7 @@ namespace realm {
  struct TableKey {
  static constexpr uint32_t null_value = uint32_t(-1) >> 1; // free top bit
  +
  constexpr TableKey() noexcept
  : value(null_value)
  {
  @@ -88,12 +89,14 @@ public:
  };
  struct ColKey {
  + static constexpr int64_t null_value = int64_t(uint64_t(-1) >> 1); // free top bit
  +
  struct Idx {
  unsigned val;
  };
  constexpr ColKey() noexcept
  - : value(uint64_t(-1) >> 1) // free top bit
  + : value(null_value) // free top bit
  {
  }
  constexpr explicit ColKey(int64_t val) noexcept
  @@ -128,7 +131,7 @@ struct ColKey {
  }
  explicit operator bool() const noexcept
  {
  - return value != ColKey().value;
  + return value != null_value;
  }
  Idx get_index() const noexcept
  {
  @@ -149,6 +152,8 @@ struct ColKey {
  int64_t value;
  };
  +static_assert(ColKey::null_value == 0x7fffffffffffffff, "Fix this");
  +
  inline std::ostream& operator<<(std::ostream& os, ColKey ck)
  {
  os << "ColKey(" << ck.value << ")";
  