fix: ValidateNested always fails with optional/nullable values even when `IsOptional({each:true})` is set

[Woodz]

Description

Minimal code-snippet showcasing the problem

class Bar {
  @IsAlphanumeric()
  baz: string;
}

class Foo {
  @IsOptional({ each: true })
  @ValidateNested({ each: true })
  bars: Map<string, Bar | null> | null;

  @IsOptional({ each: true })
  @ValidateNested({ each: true })
  abc: (Bar | null)[];
}

const foo = new Foo();
const bar1 = new Bar();
bar1.baz = "Hello";
const bar2 = new Bar();
bar2.baz = "World";
foo.bars = new Map([
  ["a", bar1],
  ["b", bar2],
  ["c", null],
]);
foo.abc = [bar1, bar2, null];

(async function () {
  console.dir(JSON.stringify(foo));
  const validationErrors = await validate(foo);
  console.dir(JSON.stringify(validationErrors));
})();

https://codesandbox.io/p/sandbox/class-validator-forked-kfxxkq?file=%2Fsrc%2Findex.ts%3A38%2C46

Expected behavior

Since we have @IsOptional({ each: true }), I expect these fields to not have any validation errors when containing nulls

Actual behavior

Returns validation errors:

1. each value in nested property bars must be either object or array
2. each value in nested property abc must be either object or array