fix array_key_exists() type error in userfiltering.php

[my-curiosity]

Hello,
I noticed that user filtering on the Reports page throws an exception with php 8:

Exception - array_key_exists(): Argument #2 ($array) must be of type array, stdClass given
Stack trace:
line 416 of /mod/hotpot/report/userfiltering.php: TypeError thrown
line 111 of /user/filters/lib.php: call to hotpot_filter_number->check_data()
line 184 of /mod/hotpot/report/renderer.php: call to user_filtering->__construct()
line 124 of /mod/hotpot/report/renderer.php: call to mod_hotpot_report_renderer->display_filters()
line 98 of /mod/hotpot/report/renderer.php: call to mod_hotpot_report_renderer->reportcontent()
line 93 of /mod/hotpot/report.php: call to mod_hotpot_report_renderer->render_report()

A quick fix done in this PR is to cast $formdata to an array in the function call. Alternatively, we could use property_exists(), which works with objects.