fix(qemu): build a command line for a headless node

_console_options() fell through the "none" branch and returned None, so
_build_command() blew up on `command.extend(None)`: a QEMU node with
console_type "none" -- offered by the GUI, and what API/MCP-driven labs use
to skip the console entirely -- failed to start with a 500 and a traceback.

Return an empty list, exactly like _aux_options() already does, and cover it
with a test that builds the command line for a headless node. Verified on a
live instance: such a node now starts (204) with `console: null` and no
-serial device in the QEMU command line.

gns3server/compute/qemu/qemu_vm.py

@@ -2042,6 +2042,7 @@ class QemuVM(KernelDatapathMixin, BaseNode):
             return self._spice_with_agent_options(self.console)
         elif self._console_type != "none":
             raise QemuError(f"Console type {self._console_type} is unknown")
+        return []
 
     def _aux_options(self):