fix batch filter with zero items

also added a test for preserving keys behavior

lib/Twig/Extension/Core.php

@@ -1488,7 +1488,7 @@ function twig_array_batch($items, $size, $fill = null)
 
     $result = array_chunk($items, $size, true);
 
-    if (null !== $fill) {
+    if (null !== $fill && !empty($result)) {
         $last = count($result) - 1;
         if ($fillCount = $size - count($result[$last])) {
             $result[$last] = array_merge(

test/Twig/Tests/Fixtures/filters/batch_with_keys.test

@@ -0,0 +1,10 @@
+--TEST--
+"batch" filter preserves array keys
+--TEMPLATE--
+{{ {'foo': 'bar', 'key': 'value'}|batch(4)|first|keys|join(',')  }}
+{{ {'foo': 'bar', 'key': 'value'}|batch(4, 'fill')|first|keys|join(',')  }}
+--DATA--
+return array()
+--EXPECT--
+foo,key
+foo,key,0,1

test/Twig/Tests/Fixtures/filters/batch_with_zero_elements.test

@@ -0,0 +1,10 @@
+--TEST--
+"batch" filter with zero elements
+--TEMPLATE--
+{{ []|batch(3)|length }}
+{{ []|batch(3, 'fill')|length }}
+--DATA--
+return array()
+--EXPECT--
+0
+0