At present, `Array.wrap` will be return an empty tuple for untyped value
because it matches to the first method_type: `(nil) -> []`.

In the type world, the behavior of an empty tuple is different from an
empty array.  For example, calling a enumerable method will cause type
an error:

```
app/app.rb:18:28: [error] Type `bot` does not have method `to_s`
│ Diagnostic ID: Ruby::NoMethod
│
└ Array.wrap(foo).map { |e| e.to_s }
                              ~~~~

```

This changes the return type of `Array.wrap` for nil value to
`Array[untyped]`.

refs: ruby#683

this also changes the case of `[T] (T ele) -> [T]` to `-> Array[T]`