improve isNull() in test

tests/unit/src/unit/issues/misc/Issue12415Abstract.hx

@@ -5,7 +5,7 @@ import haxe.Int64;
 @:fromNull
 abstract Issue12415Abstract(Int64) from Int64 to Int64 {
 	public inline function isNull() {
-		#if !static if (this == null) return true; #end
+		if (abstract == null) return true;
 		return haxe.Int64.isZero(this);
 	}
 }