Update $ne operator to include null type

Allow $ne operator to accept null values. 

null value is useful when we want to have a similar behavior as exists, but including `null` values even if the Value is not declared as nullable.

Author: christophehurpeau

src/mongo_types.ts

@@ -128,7 +128,7 @@ export interface FilterOperators<TValue> extends NonObjectIdLikeDocument {
   $in?: ReadonlyArray<TValue>;
   $lt?: TValue;
   $lte?: TValue;
-  $ne?: TValue;
+  $ne?: TValue | null;
   $nin?: ReadonlyArray<TValue>;
   // Logical
   $not?: TValue extends string ? FilterOperators<TValue> | RegExp : FilterOperators<TValue>;