See John's comment in https://bugs.openjdk.java.net/browse/JDK-8197791

... To summarize: javac should arrange a null check when it emits a cast
to a type which is statically known to be a value type, if it is compiling
at a source level which recognizes value types as such. ..

This mail: http://mail.openjdk.java.net/pipermail/valhalla-dev/2018-February/003870.html also has additional context.