TypeProf caused an infinite loop under the following situation:

```
@A = @b[0]
@b = '/' + @A
```

1. both @A and @b is untyped
2. @b is changed to a String because of the second line (String#+)
3. @A is changed to a String? because of the first line (String#[])
4. @b is changed to nil because String#+ does not accept String?
5. go to 2

This change fixes the loop by making `String#+` to accept `String?`.

TODO: it should emit a warning, but it is a bot complex considering the
situation where passing `Integer | String | nil` to the following foo:

```
def foo: (Integer) -> Integer
       | (String) -> String
``