[php-langspec][PR #229] - Change isset expression

You can view the Pull Request on github:
https://github.com/php/php-langspec/pull/229

Comment by zhujinxuan:
@nikic About trialing comma, there is another thing I am not sure how to describe in php-langspec or in php-parser.  If we have a trialing comma inside an empty array:
```
[,];
array(,);
```

In current php, it will give an Fatal error rather than a Parser error.  So I am not sure whether we shall describe it in php-langspec.