ArrayDatasource: use array_shift in fetchRow() instead of index access

Since associative arrays are supported, the numeric index 0 might not be
the first entry in the result.

library/Icinga/Data/DataArray/ArrayDatasource.php

@@ -138,7 +138,8 @@ class ArrayDatasource implements Selectable
         if (empty($result)) {
             return false;
         }
-        return $result[0];
+
+        return array_shift($result);
     }
 
     /**