sim.plot() crashes for DFN half-cell

[brosaplanella]

• PyBaMM version - $ python -m pip show pybamm: 21.9
• Python version - $ python --version: any

Describe the bug
When plotting the default plot for a simulation of a DFN half-cell model it crashes because it tries to plot the "Negative particle surface concentration [mol.m-3]" which does not exist for that model. Instead, the variables for the lithium counter electrode should be plotted (similar to how lead-acid is handled).

The key to fix this is to add a new case statement in line 163 of quick_plot.py.

To Reproduce
Steps to reproduce the behaviour:

import pybamm
import numpy as np

print(pybamm.__version__)

options = {"working electrode" : "positive"}

model = pybamm.lithium_ion.DFN(options = options)

# param = model.default_parameter_values
param = pybamm.ParameterValues(chemistry=pybamm.parameter_sets.Chen2020_plating)

sim = pybamm.Simulation(
    model,
    parameter_values=param
)
sim.solve(
    t_eval = np.linspace(0, 1000, 200),
    solver = pybamm.CasadiSolver(mode="safe"),
)
sol = sim.solution

sim.plot(output_variables=["Terminal voltage [V]"])

Expected behaviour
Plot the relevant variables for the model