Handle empty rows without E_WARNING when filling missing array indexes (#385)

In some cases, reading an XLSX file produce E_WARNING from the max()
call in the fillMissingArray() method. This commit fix the problem
by handling empty rows.

src/Spout/Reader/XLSX/Helper/CellHelper.php

@@ -31,6 +31,9 @@ class CellHelper
      */
     public static function fillMissingArrayIndexes($dataArray, $fillValue = '')
     {
+        if (empty($dataArray)) {
+            return [];
+        }
         $existingIndexes = array_keys($dataArray);
 
         $newIndexes = array_fill_keys(range(0, max($existingIndexes)), $fillValue);