Support for transformation of nullable fields

[dotslashx]

Is your feature request related to a problem? Please describe.
I am unable to use the PropertyType decorator for property which may be null or undefined.

Describe the solution you'd like
The solution might be as simple as adding the enabled option behaving exactly the same as it does in the MapFromSource decorator but independently of each other

Additional context
Simple example of the problem:

class Foo {
    @MapFromSource(obj => obj.anything)
    public something!: string;
}

class Bar {
    @PropertyType(Foo)
    @MapFromSource(obj => obj.maybeFoo)
    public foo!: Foo | null;
}

This one works as expected:

mapClasses({
    maybeFoo: {
        anything: 'whatever'
    }
}, Bar);

But this one:

mapClasses({
    maybeFoo: null
}, Bar);

Throws an error, however i would expect the result to be this:

{
    foo: null
}