Make Foo|null reflection consistent with php-src

[Ocramius]

Currently, when running reflection on an A|null type, BetterReflection produces a Roave\BetterReflection\Reflection\ReflectionUnionType:

var_dump(
    get_class(
        (new DefaultReflector(new StringSourceLocator(
            <<<'PHP'
<?php
interface A {}
final class AClass {
    private A|null $typed;
}
PHP
            ,
            (new BetterReflection())->astLocator()
        )))
            ->reflectClass('AClass')
            ->getProperty('typed')
            ->getType()
    )
);

produces

string(53) "Roave\BetterReflection\Reflection\ReflectionUnionType"

In PHP-SRC, this behavior is different: https://3v4l.org/gMA4T#v8.1rc3

<?php

interface A {}
interface B {}

class Implementation
{
    function foo(A|null $param) { throw new Exception(); }
    function bar(A|B|null $param) { throw new Exception(); }
}

var_dump((new ReflectionParameter([Implementation::class, 'foo'], 0))->getType());
var_dump((new ReflectionParameter([Implementation::class, 'bar'], 0))->getType());

produces:

object(ReflectionNamedType)#2 (0) {
}
object(ReflectionUnionType)#1 (0) {
}

This means that a UnionType AST node composed of just null plus another type should be converted into a ReflectionNamedType, for the sake of compatibility with upstream.

This is ugly, but will (for now) avoid some bad issues in downstream handling (presently blocking Roave/BackwardCompatibilityCheck#324 )