* Mark `AnyInitializer` as `ABSL_NULLABILITY_COMPATIBLE`, and constructible from
  literal `nullptr`.

  This catches constructing `absl_nonnull AnyInitializer` from an
  `absl_nullable` value (or literal `nullptr`), and constructing
  `absl_nonnull Any` from `absl_nullable AnyInitializer`.

  `AnyInitializer` cannot support `operator==` which would be consistent with
  `Any`, because it is consumable only once and the `Handle` is not known until
  it is consumed.

* Mark `Initializer` as `ABSL_NULLABILITY_COMPATIBLE`.

  This catches constructing `absl_nonnull Initializer<T>` from an
  `absl_nullable` value (or literal `nullptr`), which is meaningful when
  `T` supports nullability.

  This does not catch constructing `absl_nonnull T` from
  `absl_nullable Initializer<T>` however, because that is implemented as a
  conversion operator of `Initializer<T>` rather than a constructor of `T`.

* Remove `ABSL_NULLABILITY_COMPATIBLE` from `TypeErasedRef`, and rename its
  `== nullptr` to `empty()`.

  `TypeErasedRef` constructor takes a reference, not a pointer, and constructing
  it from `nullptr` is not empty but refers to `nullptr`. This could be
  incorrectly caught as violating nullability annotations.

  Nullability annotations at `TypeErasedRef` are not very useful anyway because
  it is normally an implementation detail of type-erasing classes, not used in
  public API.

PiperOrigin-RevId: 797165006