Add fixtures for arrow functions returning a ?? expression

[SanderMuller]

AddArrowFunctionReturnTypeRector has no fixture where the arrow function body is a ?? expression. On 2.6.4 that case got the type wrong:

fn (int $value) => $value ?? null;
// became
fn (int $value): null => $value ?? null;

Calling that throws a TypeError. Main gets all three cases below right already, so there is no source change here, just the fixtures to keep it that way:

• nullable param, ?string
• non-nullable param, int
• mixed from an untyped array, no type added

On 2.6.4 each one came out as : null.

[TomasVotruba]

Thank you 👏