Fixed php-error "undefined offset" when adding an item-convert and multiple mappings

A php error was thrown:

    Undefined offset: 0

when adding a CallbackItemConverter and after that adding multiple mappings:

```php
$workflow->addItemConverter(new CallbackItemConverter(function($item) {
    return $item;
}));

$workflow
    ->addMapping('foo', 'bar') // works
    ->addMapping('baz', 'bazzoo') // breaks
;
```
This is caused by the **getMappingItemConverter** method which accesses $converters with an index 0 instead of current
```php
$converters = \array_filter(
    $this->itemConverters,
    function ($converter) {
        return $converter instanceof MappingItemConverter;
    }
);

if (count($converters) > 0) {
    // this breaks because the MappingItemConverter is located at "1"
    // because array_filter preservers the keys
    $converter = $converters[0];
}
```