AutogenerateBookingForm not compatible with PHP 8

Unfortunately the migration AutogenerateBookingForm is not compatible with PHP 8.
Argument #2 ($replace) must be of type array|string, null given in

calendar-event-booking-bundle/src/Migration/AutogenerateBookingForm.php

Line 129 in 7bcd73d

    
           $sqlTlFormField = str_replace('##extensions##', $strExtensions, $sqlTlFormField);

The error occurs because there has been defined no default for extensions in tl_form_field since Contao 4.9 - see:
https://github.com/contao/contao/blob/4.9/core-bundle/src/Resources/contao/dca/tl_form_field.php#L283

@rabauss Please try https://github.com/markocupic/calendar-event-booking-bundle/tree/5.2.4

@markocupic: looking good to me!