websocket: fix authless

We should check if session is nullptr before referencing its data
member.

Tested:
1. build authless BMCWeb
```
meson -Drest=enabled -Dbmcweb-logging=enabled
-Dinsecure-disable-auth=enabled build && ninja -C build &&
./build/bmcweb
```
2. start websocket client without problems
```
python scripts/websocket_test.py  --host localhost:18080
```
3. bmcweb log
```
[DEBUG "websocket.hpp":221] Websocket accepted connection
[DEBUG "dbus_monitor.hpp":114] Connection opened
[DEBUG "dbus_monitor.hpp":115] Connection 0x55b22d618670 opened
[DEBUG "http_response.hpp":134] 0x55b22d611040 calling completion
handler
[DEBUG "dbus_monitor.hpp":129] Connection 0x55b22d618670 received
{"paths": ["/xyz/openbmc_project/sensors"],
"interfaces": ["xyz.openbmc_project.Sensor.Value"]}
[DEBUG "dbus_monitor.hpp":231] Creating match
type='signal',interface='org.freedesktop.DBus.Properties',
path_namespace='/xyz/openbmc_project/sensors',member='PropertiesChanged',
arg0='xyz.openbmc_project.Sensor.Value'
[DEBUG "dbus_monitor.hpp":246] Creating match
type='signal',interface='org.freedesktop.DBus.ObjectManager',
path_namespace='/xyz/openbmc_project/sensors',member='InterfacesAdded'
```

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I56613a26c129736f0e6980bb24e83f22ef60eea0