support to the case where there is no 0th array

As it stands now, this would be an error in this case.

```
$values = [
  2 => [
    'aa'
 ],
]
```

Author: YuzuruS

src/Batch.php

@@ -271,7 +271,7 @@ public function insert(Model $table, array $columns, array $values, int $batchSi
     {
         // no need for the old validation since we now use type hint that supports from php 7.0
         // but I kept this one
-        if (count($columns) !== count($values[0])) {
+        if (count($columns) !== count(current($values)) {
             return false;
         }