ScheduleExplorer will throw if a provider has a null prototype

[cavanmflynn]

I'm submitting a...

[ ] Regression 
[X] Bug report
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

The ScheduleExplorer will throw an unhandled promise rejection upon module initialization if the discovery service returned any providers that have a null prototype.

The provider will make it past the instance check and the error will ultimately be thrown inside the getAllFilteredMethodNames method when attempting to access properties of the null prototype:

(node:81446) UnhandledPromiseRejectionWarning: TypeError: Cannot convert undefined or null to object
    at Function.getOwnPropertyNames (<anonymous>)
    at MetadataScanner.getAllFilteredMethodNames (/source/project/node_modules/@nestjs/core/metadata-scanner.js:24:45)
    at getAllFilteredMethodNames.next (<anonymous>)
    at new Set (<anonymous>)
    at MetadataScanner.scanFromPrototype (/source/project/node_modules/@nestjs/core/metadata-scanner.js:9:29)
    at /source/project/node_modules/@nestjs/schedule/dist/schedule.explorer.js:36:34
    at Array.forEach (<anonymous>)
    at ScheduleExplorer.explore (/source/project/node_modules/@nestjs/schedule/dist/schedule.explorer.js:30:19)
    at ScheduleExplorer.onModuleInit (/source/project/node_modules/@nestjs/schedule/dist/schedule.explorer.js:26:14)
    at MapIterator.iteratee (/source/project/node_modules/@nestjs/core/hooks/on-module-init.hook.js:21:43)
    at MapIterator.next (/source/project/node_modules/iterare/src/map.ts:9:39)
    at IteratorWithOperators.next (/source/project/node_modules/iterare/src/iterate.ts:19:28)
    at Function.from (<anonymous>)
    at IteratorWithOperators.toArray (/source/project/node_modules/iterare/src/iterate.ts:227:22)
    at callOperator (/source/project/node_modules/@nestjs/core/hooks/on-module-init.hook.js:22:10)
    at Object.callModuleInitHook (/source/project/node_modules/@nestjs/core/hooks/on-module-init.hook.js:42:23)

Expected behavior

The ScheduleExplorer gracefully handles a provider with a null prototype. Could be as simple as returning early if the prototype doesn't exist:

if (!instance || !Object.getPrototypeOf(instance)) {
  return;
}

I can open a PR with the above change if it's acceptable.

Minimal reproduction of the problem with instructions

This is reproducible using any provider created with Object.create(null). In my case, it's a custom provider that consists of a proxy where the target is an object created with Object.create(null).

What is the motivation / use case for changing the behavior?

Environment

Nest version:
@nestjs/common: 7.0.2
@nestjs/core: 7.0.2
@nestjs/schedule: 0.3.0
 
For Tooling issues:
- Node version: v12.16.1
- Platform: Mac

Others:

[kamilmysliwiec]

Thanks for reporting! Would you like to create a PR for this issue?

[cavanmflynn]

Will do 👍

[kamilmysliwiec]

Merged! Thanks