AutoBuilder with a generic parameter should be parametric in underlying type's nullability

Consider this use of AutoBuilder:

```java
@AutoBuilder(callMethod = "of")
public abstract static class Builder {

@Override
public abstract Builder setValue(T value);

...
}
```
(distilled from https://github.com/apache/beam/pull/34902 but I will likely have changed it with a workaround by the time you click)

The type parameter `T` can legitimately be instantiated with any nullability: `@NonNull String`, `@Nullable String`, etc. But the generated builder class throws `NullPointerException` in response to `setValue(null)`. Instead, the builder class should be fully agnostic as to the nullability (or other qualifiers) for type `T`.