This commit fixes the following PHPStan errors introduced after the
update to version 0.12.83:
```
------ ----------------------------------------------------------------------
Line lib/Listing/Handler.php
------ ----------------------------------------------------------------------
18 Parameter #1 $callback of function call_user_func_array expects
callable(): mixed, array(class-string|object, 'listingQuery') given.
45 Parameter #1 $callback of function call_user_func_array expects
callable(): mixed, array(class-string|object, 'groups') given.
54 Parameter #1 $callback of function call_user_func_array expects
callable(): mixed, array(class-string|object, 'filters') given.
62 Parameter #1 $callback of function call_user_func_array expects
callable(): mixed, array(class-string|object, 'listingQuery') given.
------ ----------------------------------------------------------------------
```
It was necessary to use both is_callable() and method_exists() instead
of just is_callable(), as a parent class of the model classes implements
__call() and thus is_callable() always returns true even when the
method doesn't exist (and in those cases method_exists() returns false).
[MAILPOET-3491]
3d31f1f030