grapheme_substr inconsistency as compared to substr

`grapheme_substr` from Intl have differing behavior as compared to `substr`:
```php
var_dump(substr('a', 1, null)); //  string(0) ""`
var_dump(grapheme_substr('a', 1, null)); // bool(false)
```
It looks like on php 5.x it also produces warning. 

This behavior needs to be investigated against other wrappers and corrected if needed.
