Fix change type mapping for nullable links

Author: CarsonF

src/core/database/changes.ts

@@ -65,11 +65,15 @@ type ChangeKey<Key extends keyof T & string, T> = T[Key] extends SetChangeType<
 
 type ChangeOf<Val> = Val extends SetChangeType<any, infer Override>
   ? Override
-  : UnwrapSecured<Val> extends FileId
+  :
+      | RawChangeOf<UnwrapSecured<Val> & {}>
+      | (UnwrapSecured<Val> extends null ? null : unknown);
+
+type RawChangeOf<Val> = Val extends FileId
   ? CreateDefinedFileVersionInput
-  : UnwrapSecured<Val> extends LinkTo<any>
+  : Val extends LinkTo<any>
   ? ID
-  : UnwrapSecured<Val>;
+  : Val;
 
 /**
  * Only props of T that can be written directly to DB