(v 3.2) Fix the installer to allow subfolders

Problem/Motivation

Right now, there may be a problem with installing the site inside a path, like https://example.com/drupal. The installer is not compatible with such solution. This was reported in https://github.com/droptica/droopler_project/issues/137

Steps to reproduce

I've started with
composer create-project droptica/droopler-project droopler8310 "^8.3.1"
(this is for you to know the profile version)

Then I followed usual steps (and that's not the first time I install Droopler).
Everything worked well till the site install.
At site install, during the "Install additional components" step, I've got an error which is:
n AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /droopler/core/install.php?rewrite=ok&profile=droopler&langcode=en&id=2&op=do_nojs&op=do
StatusText: OK
ResponseText: Drupal\Core\Database\InvalidQueryException: Query condition 'nfd.nid IN ()' cannot be empty. in Drupal\Core\Database\Query\Condition->condition() (line 117 of C:\Data\webdev\drupalProjects\droopler8310\web\core\lib\Drupal\Core\Database\Query\Condition.php).