[php-src] Issue #21507: Confusing error message

Issue: https://github.com/php/php-src/issues/21507
Author: exakat

### Description

The following code:

```php
<?php

class X {
  const A = '1';
}
$c = 'b'; 
isset(X::{$c}); 

```

Resulted in this (valid) output:
```
atal error: Cannot use isset() on the result of an expression (you can use "null !== expression" instead)
```

but I cannot use ``null !== X::{$c}``, which also yields a Fatal Error.  

With a constant, this message should recommend using 'defined()' instead, or on top of the current recommendation. 

### PHP Version

```plain
Tested on PHP 8.5, but any version since 8.3
```

### Operating System

Not applicable.