New check request: autoboxing on ternary operator

Imagine I have this method:

```java
Boolean returnABoolean()
```

and:

```java
boolean isEnabled = true;
Boolean myValue = isEnabled ? returnABoolean() : false;
```

This will crash if the `returnABoolean()` returns `null` because the [JLS says](https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.25.1):

> If the second and third operands are both of type Boolean, the conditional expression has type Boolean. **Otherwise, the conditional expression has type boolean.**

If ErrorProne could detect this it would be very helpful.