Enforce 1:0..1 relationship with EF6 Database First - sql-server

Question

In my model, I have three tables, Storage, Haul and StoragePlace. One storage can either be a place or haul. For the StoragePlace, I already achieved a 1:0..1 relationship. The problem is the Haul entity.
CREATE TABLE dbo.Haul
(
ID INT NOT NULL PRIMARY KEY IDENTITY,
ID_Storage INT NOT NULL UNIQUE,
Arrival DATETIME NOT NULL DEFAULT getdate(),
Depature DATETIME NULL,
CONSTRAINT FK_Haul_to_Storage FOREIGN KEY (ID_Storage) REFERENCES Storage(ID) ON DELETE CASCADE
)
GO
/** --- See edit below --
CREATE UNIQUE NONCLUSTERED INDEX UQX_Storage
ON Haul(ID_Storage)
WHERE ID_Storage IS NOT NULL;
GO
**/
--------------------------------------------------
CREATE TABLE dbo.Storage
(
ID INT NOT NULL PRIMARY KEY IDENTITY,
Description1 NVARCHAR(100) NULL,
Description2 NVARCHAR(100) NULL,
AddedBy NVARCHAR(50) NOT NULL
)
--------------------------------------------------
CREATE TABLE dbo.StoragePlace
(
ID INT NOT NULL PRIMARY KEY,
ID_PlantArea INT NOT NULL,
CONSTRAINT FK_StoragePlace_to_Storage FOREIGN KEY (ID) REFERENCES Storage(ID) ON DELETE CASCADE,
CONSTRAINT FK_StoragePlace_to_PlantArea FOREIGN KEY (ID_PlantArea) REFERENCES PlantArea(ID) ON DELETE CASCADE
)
My thought was, that UQX_Storage would trigger EF6 to accept a 1:0..1 relationship between Haul and Storage. Certainly, this doesn't work; it still insists on a 1:* relationship, which, from my understanding, shall not be possible with this model above.
How can I enforce the intended 1:0..1 relationship between Haul and Storage?
EDIT:
I just realized, that the ID_Storage field can never be NULL, so I replaced the UQX_Storage "constraint" with a simple ID_Storage INT NOT NULL UNIQUE,. However, that didn't solve the problem either.