php returns NULL on invalid input

```bash
$ php -r '$x = hypot([], "a"); var_dump($x);'
PHP Warning:  hypot() expects parameter 1 to be double, array given in Command line code on line 1

Warning: hypot() expects parameter 1 to be double, array given in Command line code on line 1
NULL
```

Author: kvz

functions/math/hypot.js

@@ -4,7 +4,7 @@ function hypot(x, y) {
   //   example 1: hypot(3, 4);
   //   returns 1: 5
   //   example 2: hypot([], 'a');
-  //   returns 2: 0
+  //   returns 2: null
 
   return Math.sqrt(x * x + y * y) || 0;
-}
+}