[libinput] Re-run touchscreen->output mapping when externally clearing output name

When setting a device's output name externally we assign the relevant output to the device

If we pass an empty name we set a null output, which causes a crash the next time the touchscreen is touched

Since passing an empty name semantically means 'Detect output automatically' rerun the mapping heuristic so we get a valid output

src/backends/libinput/connection.cpp

@@ -277,6 +277,14 @@ void Connection::processEvents()
             applyDeviceConfig(device);
             applyScreenToDevice(device);
 
+            connect(device, &Device::outputNameChanged, this, [this, device] {
+                // If the output name changes from something to empty we need to
+                // re-run the assignment heuristic so that an output is assinged
+                if (device->outputName().isEmpty()) {
+                    applyScreenToDevice(device);
+                }
+            });
+
             Q_EMIT deviceAdded(device);
             break;
         }