Delete php-bcmath and try to perform frontend installation "frontends/php/setup.php", use case is new installation

Expected:
Warns that php-bcmath is missing

Actual:
Empty setup page and the error in log file:

PHP Fatal error:  Uncaught Error: Call to undefined function bccomp() in /var/www/html/zabbix/frontends/php/include/classes/setup/CFrontendSetup.php:123

Notice that since ZBX-15464 it is no longer possible to perform setup if
php-bcmath is missing due to bccomp being used in setup process itself before checking if it exists.

This results in following error in the log and empty setup page, also strange code to check bcmatch existence after it was already used and misleading documentation that does mention that it is required to install php-bcmath before starting installation.