fix typings for PartialModelGraph and nullable fields

Handle `| null` unions properly. (e.g. for nullable `BelongsToOneRelations`).

Before `PartialModelGraph` was converting `field: Model | null;` to `field?: Expression<Model | null>`, and typescript throws errors for graphs with partial sub-graphs for those nullable relations. (expecting full instance of the model class)
Now it converts it properly to `field?: PartialModelGraph<Model> | null` 🎉

typings/objection/index.d.ts

@@ -225,16 +225,20 @@ declare namespace Objection {
    */
   type PartialModelGraph<M, T = M & GraphParameters> = T extends any
     ? {
-        [K in DataPropertyNames<T>]?: Defined<T[K]> extends Model
-          ? PartialModelGraph<Defined<T[K]>>
-          : Defined<T[K]> extends Array<infer I>
-          ? I extends Model
-            ? PartialModelGraph<I>[]
-            : Expression<T[K]>
-          : Expression<T[K]>;
+        [K in DataPropertyNames<T>]?: null extends T[K]
+          ? PartialModelGraphField<NonNullable<T[K]>> | null // handle nullable BelongsToOneRelations
+          : PartialModelGraphField<T[K]>;
       }
     : never;
 
+  type PartialModelGraphField<F> = Defined<F> extends Model
+    ? PartialModelGraph<Defined<F>>
+    : Defined<F> extends Array<infer I>
+      ? I extends Model
+        ? PartialModelGraph<I>[]
+        : Expression<F>
+      : Expression<F>;
+
   /**
    * Extracts the property names (excluding relations) of a model class.
    */