Check for valid _foregroundProcessInfo to prevent crash

Summary:
In Session::getUrl() check that _foregroundProcessInfo is valid before
tyring to access its members.

I couldn't reproduce the crash from the BR, but it is logical to call
isValid() before accessing any ProcessInfo object members.

BUG: 391447

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D12175