[BUG] fix(AdminUrlGenerator): unitialize before early returns

[JacquesDurand]

Hi ! I noticed that trying to upgrade after 4.16.1 (and this commit I believe dc4bc4c ) while still not using pretty urls (we intend to, but it is a long journey ^^), some of the generated urls break.

For instance, the url for filter modal becomes htps://localhost/admin?crudAction=renderFilters. Every other param disappears.
I noticed that the previous generated url was one for the dashboard with then went trough this code bloc :

        if ([] === $routeParameters) {
            return $this->urlGenerator->generate($this->dashboardRoute);
        }

I figured the previous AdminUrlGenerator was then not uninitialized, which might leak wrong params to the next generated url.

This seems at least to fix my issue, but I might be missing something, WDYT ?