fix(model): ignore falsy last argument to create() for backwards compatibility

lib/model.js

@@ -2812,19 +2812,27 @@ Model.create = async function create(doc, options) {
   } else {
     const last = arguments[arguments.length - 1];
     options = {};
-    if (typeof last === 'function' || (arguments.length > 1 && !last)) {
-      if (typeof options === 'function' ||
-          typeof arguments[2] === 'function') {
-        throw new MongooseError('Model.create() no longer accepts a callback');
-      }
+    const hasCallback = typeof last === 'function' ||
+      typeof options === 'function' ||
+      typeof arguments[2] === 'function';
+    if (hasCallback) {
+      throw new MongooseError('Model.create() no longer accepts a callback');
     } else {
       args = [...arguments];
+      // For backwards compatibility with 6.x, because of gh-5061 Mongoose 6.x and
+      // older would treat a falsy last arg as a callback. We don't want to throw
+      // an error here, because it would look strange if `Test.create({}, void 0)`
+      // threw a callback error. But we also don't want to create an unnecessary document.
+      if (args.length > 1 && !last) {
+        args.pop();
+      }
     }
 
     if (args.length === 2 &&
         args[0] != null &&
         args[1] != null &&
         args[0].session == null &&
+        last &&
         getConstructorName(last.session) === 'ClientSession' &&
         !this.schema.path('session')) {
       // Probably means the user is running into the common mistake of trying

test/model.create.test.js

@@ -166,6 +166,14 @@ describe('model', function() {
             assert.ok(doc._id);
           });
       });
+
+      it('ignores undefined last arg (gh-13487)', async function() {
+        await B.deleteMany({});
+        await B.create({ title: 'foo' }, void 0);
+        const docs = await B.find();
+        assert.equal(docs.length, 1);
+        assert.equal(docs[0].title, 'foo');
+      });
     });
   });
 });