Process key event even without active TerminalDisplay

Summary:
There are (more or less) valid cases when currentTerminalDisplay can be
not set:
* konsole started with `--background-mode`, `sendText` called via D-Bus
* text send through KPart's `sendInput()`

It can be assumed read-only is not set in this case.

This patch should also fix TerminalInterfaceTest. Currently the shell
probably doesn't get commands send through sendInput.

Test Plan:
* Run `konsole --background-mode --nofork & ; sleep 2; qdbus org.kde.konsole-$! /Sessions/1 sendText $'echo test\n'`
* Press Ctrl+Shift+F12
* The terminal should show `echo test` command and its result

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D11762

(cherry picked from commit c0a46e2b)