bugfix: the Language to be set MUST not be NULL (ILIAS-eLearning#4799)

otherwise a TypeError Exception is raised:

TypeError thrown with message "Argument 1 passed to ilObjUser::setLanguage() must be of the type string, null given, called in /.../Services/User/classes/class.ilUserImportParser.php on line 467"

Stacktrace:
#16 TypeError in /.../Services/User/classes/class.ilObjUser.php:1479
#15 ilObjUser:setLanguage in /.../Services/User/classes/class.ilUserImportParser.php:467
#14 ilUserImportParser:importBeginTag in /.../Services/User/classes/class.ilUserImportParser.php:364
#13 ilUserImportParser:handlerBeginTag in [internal]:0

Co-authored-by: Uwe Tesche <[email protected]>