fix: @ValidateNested does not work recursively

[jangxx]

Description

When using @ValidateNested() on a class that's nested more than one level, the validation passes class instances with missing properties.

Minimal code-snippet showcasing the problem

import { IsString, ValidateNested, validateOrReject } from "class-validator";
import { plainToInstance, Type } from "class-transformer";
import "reflect-metadata";

class Level3 {
	@IsString()
	aString: string;
}

class Level2 {
	@Type(() => Level3)
	@ValidateNested()
	level3: Level3;
}

class Level1 {
	@Type(() => Level2)
	@ValidateNested()
	level2: Level2;
}

const instance = plainToInstance(Level1, { level2: {} });

validateOrReject(instance);

console.log(instance);

Expected behavior

This snippet should throw an error, since {} clearly isn't a valid Level2 class.

Actual behavior

The validation passes and the output reads Level1 { level2: Level2 {} }. Adding @IsDefined() to the level3 property in the Level2 class makes the validator throw an error as expected. Similarly, adding another property with a decorator like @IsString() someString: string will also correctly fail the validation, because the key is missing. But for some reason, the level3 property with only the @ValidateNested() decorator is ignored and validation passes.

[jangxx]

Actually, changing the the plainToInstance line to const instance = plainToInstance(Level1, { }); also passes validation, even though an empty object shouldn't pass as a valid Level1 class instance either.