Create table with Nullable foreign keys in SQLite

With this orderitem table configuration (where user collects order products) i cant insert value correctly because as i said the same field Codart is in both table product and variants

So how can define the fk so the table accept product with and without a variant

I repeat: table product has all products (all codart values)

instead table variants has only products  (codart) that has at least a variant (codvar);

Variant table not has for example a product that has no variant and that is stored in Product table but not in Variant

B1 is in table Variant with variants 1 and 2

So fk in Product (codart) not permit to add B1 with variants

CREATE TABLE OrderItem
(
  
  Codart TEXT NOT NULL,
  Codvar TEXT NOT NULL DEFAULT "",
  Descri TEXT NOT NULL,
  Quanty INT NOT NULL,
  Price REAL NOT NULL,
  ...

  CONSTRAINT PK_Riorcl PRIMARY KEY(...Codart,Codvar),
  CONSTRAINT FK_Product FOREIGN KEY (Codart) REFERENCES Product(Codart),
  CONSTRAINT FK_Variants FOREIGN KEY (Codart, Codvar) REFERENCES Variants(Codart,Codvar)    
)