While the specification for `Throwable::getMessage` allows for a null message, it seems odd that the Math XXXExact intrinsics throw an ArithmeticException with a null message.

This is a change in behaviour between JDK 24 and JDK 25.

```
$ cat TestMathAddExactOverflow.java
public class TestMathAddExactOverflow {

 public static void main(String... args) {
    for (int i = 0; i < 20_000; i++) {
      addExactImpl(Integer.MAX_VALUE, i);
    }
  }

  static int addExactImpl(int a, int b) {
    try {
      return Math.addExact(a, b);
    } catch (ArithmeticException e) {
      if (e.getMessage() == null) {
        var x = new AssertionError("null message, b=" + b);
        x.addSuppressed(e);
        throw x;
      }
   }
   return -1;
  }
}
```

$ ./jdk-24..0.2.jdk/Contents/Home/bin/java TestMathAddExactOverflow

$ ./jdk-25.jdk/Contents/Home/bin/java -XX:+UnlockDiagnosticVMOptions -XX:DisableIntrinsic=_addExactI TestMathAddExactOverflow

$ ./jdk-25.jdk/Contents/Home/bin/java TestMathAddExactOverflow
Exception in thread "main" java.lang.AssertionError: null message, b=7745
at TestMathAddExactOverflow.addExactImpl(TestMathAddExactOverflow.java:14)
at TestMathAddExactOverflow.main(TestMathAddExactOverflow.java:5)
Suppressed: java.lang.ArithmeticException

Math:: subtractExact shows similar behaviour too.