config/actions: fix null derefs in pin dispatcher (#14914)

* hyprtester: add issue14134 regression test

D#14134 reports a pin dispatcher SEGV when a monitor hosting a
pinned floating window is disabled (e.g. lid close on a laptop).
After CMonitor::onDisconnect resets m_activeWorkspace to null,
the CMonitor SP stays alive in m_realMonitors, and the pinned-
window branch of moveWorkspaceToMonitor leaves the window's
m_monitor pointing at the disabled monitor.

The next pin dispatch does null deref inside Config::Actions::pinWindow.

* config/actions: fix null derefs in pin dispatcher

PR #14061 replaced moveToWorkspace(m_activeWorkspace) with
assignToSpace(m_activeWorkspace->m_space) and a separate
layoutTarget() call.

The two new derefs aren't covered by the existing PMONITOR check,
and either one being null crashes the compositor.

In practice, i use pyprland which uses pining function,
and since i am on laptop, sometimes my lid is closed and i have
monitor disabled, this causes null deref when computer goes idle.

    Layout::CWindowTarget::assignToSpace   <- segv
    Config::Actions::pinWindow
    pin
    CDispatcherTranslator::run
    CHyprCtl::getReply

In this fix i've added null checks to avoid crashing