Fix: aggregate not matching null values

spec/ParseQuery.Aggregate.spec.js

@@ -941,6 +941,52 @@ describe('Parse.Query Aggregate testing', () => {
       });
   });
 
+  it_exclude_dbs(['postgres'])('match null values', async () => {
+    const obj1 = new Parse.Object('MyCollection');
+    obj1.set('language', 'en');
+    obj1.set('otherField', 1);
+    const obj2 = new Parse.Object('MyCollection');
+    obj2.set('language', 'en');
+    obj2.set('otherField', 2);
+    const obj3 = new Parse.Object('MyCollection');
+    obj3.set('language', null);
+    obj3.set('otherField', 3);
+    const obj4 = new Parse.Object('MyCollection');
+    obj4.set('language', null);
+    obj4.set('otherField', 4);
+    const obj5 = new Parse.Object('MyCollection');
+    obj5.set('language', 'pt');
+    obj5.set('otherField', 5);
+    const obj6 = new Parse.Object('MyCollection');
+    obj6.set('language', 'pt');
+    obj6.set('otherField', 6);
+    await Parse.Object.saveAll([obj1, obj2, obj3, obj4, obj5, obj6]);
+
+    expect(
+      (await new Parse.Query('MyCollection').aggregate([
+        {
+          match: {
+            language: { $in: [null, 'en'] },
+          },
+        },
+      ]))
+        .map(value => value.otherField)
+        .sort()
+    ).toEqual([1, 2, 3, 4]);
+
+    expect(
+      (await new Parse.Query('MyCollection').aggregate([
+        {
+          match: {
+            $or: [{ language: 'en' }, { language: null }],
+          },
+        },
+      ]))
+        .map(value => value.otherField)
+        .sort()
+    ).toEqual([1, 2, 3, 4]);
+  });
+
   it('project query', done => {
     const options = Object.assign({}, masterKeyOptions, {
       body: {

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -841,7 +841,9 @@ export class MongoStorageAdapter implements StorageAdapter {
   // As much as I hate recursion...this seemed like a good fit for it. We're essentially traversing
   // down a tree to find a "leaf node" and checking to see if it needs to be converted.
   _parseAggregateArgs(schema: any, pipeline: any): any {
-    if (Array.isArray(pipeline)) {
+    if (pipeline === null) {
+      return null;
+    } else if (Array.isArray(pipeline)) {
       return pipeline.map(value => this._parseAggregateArgs(schema, value));
     } else if (typeof pipeline === 'object') {
       const returnValue = {};