Fixed contract in empty array

This problems are:

## case1
```perl
my $itr = Iterator::GroupedRange->new([]);
$itr->next; # returned []
```
But I want __empty__.

## case2
```perl
my $itr = Iterator::Grouped::Range->new([]);
$itr->append([1..10]);
$itr->has_next; # returned false
```
But I want __true__.