[php-langspec][Issue #227] - Array initializer does not include in ','

You can view the Issue on github:
https://github.com/php/php-langspec/issues/227

Opened By: zhujinxuan
Issue Description:
In current PHP, the following code is legal:
```
$a = [,];
```

And the following code is legal in parser but illegal during runtime:
```
$a = [,,];
```
However, current `array-initializer` in `10-expressions.md` and `19-grammar.md` does not include `,` as an element. 

If we want to describe this behavior in the langspec, then we need to change the definition of `array-initializer` to 
```
array-initializer:
  array-initializer-list ','?
  ','
```

I can submit a PR if you think it is good to support this current behavior.