empty panel name => guake fails to start

**Describe the bug**

Just accidentally renamed tab to "rust" then tried to reset it and set it to empty string "".
After guake restart it failed. Tried in another terminal to get error output:

```Guake not running, starting it
[WARN] ===================================================================
[WARN] Unable to load the library libutempter !
[WARN] Some feature might not work:
[WARN] - 'exit' command might freeze the terminal instead of closing the tab
[WARN] - the 'wall' command is known to work badly
[WARN] Error: libutempter.so.0:
[WARN] ===================================================================²
Loading Gnome schema from: /usr/share/glib-2.0/schemas
Language previously loaded from: /usr/lib/python3.11/site-packages/guake/po
Guake Terminal 3.9.0
VTE 0.72.2
Gtk 3.24.38
created fresh notebook for workspace 0
Spawning new terminal at /home/user
current workspace is 0
can't bind show-focus key
Spawning new terminal at /home/user
Traceback (most recent call last):
File "/usr/lib/python3.11/site-packages/dbus/bus.py", line 173, in activate_name_owner
return self.get_name_owner(bus_name)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/dbus/bus.py", line 348, in get_name_owner
return self.call_blocking(BUS_DAEMON_NAME, BUS_DAEMON_PATH,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/dbus/connection.py", line 634, in call_blocking
reply_message = self.send_message_with_reply_and_block(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.NameHasNoOwner: Could not get owner of name 'org.guake3.RemoteControl': no such name

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3.11/site-packages/guake/main.py", line 473, in main
remote_object = bus.get_object(DBUS_NAME, DBUS_PATH)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/dbus/bus.py", line 237, in get_object
return self.ProxyObjectClass(self, bus_name, object_path,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/dbus/proxies.py", line 250, in __init__
self._named_service = conn.activate_name_owner(bus_name)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/dbus/bus.py", line 178, in activate_name_owner
self.start_service_by_name(bus_name)
File "/usr/lib/python3.11/site-packages/dbus/bus.py", line 273, in start_service_by_name
return (True, self.call_blocking(BUS_DAEMON_NAME, BUS_DAEMON_PATH,
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/dbus/connection.py", line 634, in call_blocking
reply_message = self.send_message_with_reply_and_block(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
dbus.exceptions.DBusException: org.freedesktop.DBus.Error.ServiceUnknown: The name org.guake3.RemoteControl was not provided by any .service files

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/bin/guake", line 8, in
sys.exit(exec_main())
^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/guake/main.py", line 648, in exec_main
if not main():
^^^^^^
File "/usr/lib/python3.11/site-packages/guake/main.py", line 489, in main
instance = Guake()
^^^^^^^
File "/usr/lib/python3.11/site-packages/guake/guake_app.py", line 269, in __init__
self.restore_tabs(suppress_notify=True)
File "/usr/lib/python3.11/site-packages/guake/guake_app.py", line 1483, in restore_tabs
box, page_num, term = nb.new_page_with_focus(
^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/guake/notebook.py", line 418, in new_page_with_focus
self.rename_page(page_num, self.guake.compute_tab_title(terminal), False)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/guake/guake_app.py", line 1127, in compute_tab_title
guake_yml = self.load_cwd_guake_yaml(vte)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/site-packages/guake/guake_app.py", line 1111, in load_cwd_guake_yaml
cwd = Path(vte.get_current_directory())
^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get_current_directory'
```

digged deeper and in session.js
```
"label": "",
"custom_label_set": true
```

setting label to anything except empty string solved problem.

**Expected behavior**

guake should run even if label is emtpy or don't allow to empty it

**Actual behavior**

guake fails to start

**To Reproduce**

set panel label to empty