[java] -java-lib should parse @Nullable for proper Null-safety work

Hello! :)

Would be nice if `-java-lib` may parse Java's `@Nullable` annotation on function arguments.
For example, Java's method:
```java
public void setMapStyle(@Nullable MapStyleOptions options)
```
Is parsed as:
```haxe
public function setMapStyle(options:MapStyleOptions):Void;
```
intesad of:
```haxe
public function setMapStyle(options:Null):Void;
```

Which is not very well if you using the new null-safety feature.

Thank you!