[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 nikic:
@zhujinxuan Ah, this case is more complicated, because generally `,` is allowed inside list, e.g. you can write `list($a,,,,,$b)`. What is not allowed is a list that consists *only* of `,`, such as `list(,,,,,)`. This kind of rule is best noted in the "Constraints" section, rather than the grammar.