Parse *times out* if invalid Pointer is sent with a beforeSave trigger in place

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version of Parse Server.

Issue Description

If a beforeSave is deployed for a certain class (even if it's just an empty function), trying to create a new object (POST) with a malformed Pointer (objectId is not present or is not a String or is an empty string) produces a timeout.

If the beforeSave is not there, even with a malformed Pointer, the object is created correctly.

Steps to reproduce

1. Create a class named Stuff with a field named Owner of type Pointer and set the target class to User
2. Deploy a main.js file with a beforeSave for the Stuff class: Parse.Cloud.beforeSave('Stuff', (request) => {});
3. Try to POST with a malformed Pointer in the Owner field:

curl 'https://YOUR.PARSE-SERVER.HERE/parse/classes/Stuff' 
-H 'X-Parse-Application-Id: <App-ID>' 
-H 'X-Parse-Client-Key: <Client-Key>' 
--data-raw '{"Owner":{"__type":"Pointer","className":"_User","obejctId":""}}'

* note that in this request objectId is an empty string

Actual Outcome

No response, eventually times out.

Expected Outcome

Successful creation of the new object, as it does without the beforeSave:
{"objectId":"22IlcgdXQO","createdAt":"2021-08-14T22:25:33.899Z"}

Failing Test Case / Pull Request

• 🤩 I submitted a PR with a fix and a test case.
• 🧐 I submitted a PR with a failing test case.

Environment

Server

• Parse Server version: 4.5.0
• Operating system: unknown
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): Back4App

Database

• System (MongoDB or Postgres): MongoDB
• Database version: 3.6
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): Back4App

Client

• SDK (iOS, Android, JavaScript, PHP, Unity, etc): REST
• SDK version: unknown

Logs

2021-08-14T22:57:59.757Z -
(node:18) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 5)
2021-08-14T22:57:59.756Z -
(node:18) UnhandledPromiseRejectionWarning: Error: Cannot create a pointer to an unsaved ParseObject
    at ParseUser.toPointer (/usr/src/app/node_modules/parse-server/node_modules/parse/lib/node/ParseObject.js:813:13)
    at encode (/usr/src/app/node_modules/parse-server/node_modules/parse/lib/node/encode.js:64:20)
    at _default (/usr/src/app/node_modules/parse-server/node_modules/parse/lib/node/encode.js:132:10)
    at SetOp.toJSON (/usr/src/app/node_modules/parse-server/node_modules/parse/lib/node/ParseOp.js:155:32)
    at ParseObject._getSaveJSON (/usr/src/app/node_modules/parse-server/node_modules/parse/lib/node/ParseObject.js:490:37)
    at success (/usr/src/app/node_modules/parse-server/lib/triggers.js:385:45)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)

Reproduce the POST described in the issue with a beforeSave trigger and malformed Pointer, then inspect the success path in lib/triggers.js, especially the area shown in the stack trace. The issue is resolved when this request no longer times out or produces an unhandled rejection and matches the successful creation behavior without the trigger.