fix(document): ensure calling get() with empty string returns undefined for mongoose-plugin-autoinc

Fix #9681

Author: vkarpov15

package.json

@@ -24,7 +24,7 @@
     "kareem": "2.3.2",
     "mongodb": "3.6.3",
     "mongoose-legacy-pluralize": "1.0.2",
-    "mpath": "0.8.0",
+    "mpath": "0.8.1",
     "mquery": "3.2.2",
     "ms": "2.1.2",
     "regexp-clone": "1.0.0",

test/document.test.js

@@ -9805,4 +9805,13 @@ describe('document', function() {
       assert.equal(err.message, 'test err');
     });
   });
+
+  it('returns undefined rather than entire object when calling `get()` with empty string (gh-9681)', function() {
+    const TestSchema = new Schema({ name: String });
+    const TestModel = db.model('Test', TestSchema);
+
+    const testObject = new TestModel({ name: 't' });
+
+    assert.strictEqual(testObject.get(''), void 0);
+  });
 });