Skip DontCrashEmptyDecorationTest if we don't have a /dev/dri/card0 device

Without the /dev/dri/card0 mesa fails to initialize egl and the test
fails. In order to silence the failure till the system provides the
device file, we better skip the test.

A nullptr crash in that case is fixed in WaylandServer tear-down.

autotests/wayland/dont_crash_empty_deco.cpp

@@ -70,6 +70,9 @@ private:
 
 void DontCrashEmptyDecorationTest::initTestCase()
 {
+    if (!QFile::exists(QStringLiteral("/dev/dri/card0"))) {
+        QSKIP("Needs a dri device");
+    }
     qRegisterMetaType<KWin::ShellClient*>();
     qRegisterMetaType<KWin::AbstractClient*>();
     QSignalSpy workspaceCreatedSpy(kwinApp(), &Application::workspaceCreated);

wayland_server.cpp

@@ -97,9 +97,11 @@ void WaylandServer::terminateClientConnections()
 {
     destroyInternalConnection();
     destroyInputMethodConnection();
-    const auto connections = m_display->connections();
-    for (auto it = connections.begin(); it != connections.end(); ++it) {
-        (*it)->destroy();
+    if (m_display) {
+        const auto connections = m_display->connections();
+        for (auto it = connections.begin(); it != connections.end(); ++it) {
+            (*it)->destroy();
+        }
     }
 }