Allow null 'format' in DateRangeAggregation

[pmishev]

Allow null 'format' in DateRangeAggregation

Without this, calling:

        $ranges[] = ['from' => time()-1000, 'to' => time(), 'key' => 'time_range'];
        $rangeAgg = new DateRangeAggregation('myagg', 'created', null, $ranges, true);

results in an exception, but it shouldn't as format is optional.