Fix bug with side by side plotting of signal containing navigation dimension only

[ericpre]

Progress of the PR

• Fix bug described in title,
• [n/a] update docstring (if appropriate),
• [n/a] update user guide (if appropriate),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

import hyperspy.api as hs
import matplotlib
import numpy as np

matplotlib.use("module://ipympl.backend_nbagg")
s = hs.signals.Signal2D(np.random.random((2, 2))).T
s.plot()

Give the following error:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[5], line 2
      1 s = hs.signals.Signal2D(np.random.random((2, 2))).T
----> 2 s.plot()

File ~\Dev\hyperspy\hyperspy\signal.py:2954, in BaseSignal.plot(self, navigator, axes_manager, plot_markers, **kwargs)
   2949     else:
   2950         raise ValueError(
   2951             'navigator must be one of "spectrum","auto", '
   2952             '"slider", None, a Signal instance')
-> 2954 self._plot.plot(**kwargs)
   2955 self.events.data_changed.connect(self.update_plot, [])
   2957 p = self._plot.signal_plot if self._plot.signal_plot else self._plot.navigator_plot

File ~\Dev\hyperspy\hyperspy\drawing\mpl_he.py:220, in MPL_HyperExplorer.plot(self, **kwargs)
    218     import matplotlib.pyplot as plt
    219     with plt.ioff():
--> 220         plot_sig_and_nav(plot_style)
    221 else:
    222     plot_sig_and_nav(plot_style)

File ~\Dev\hyperspy\hyperspy\drawing\mpl_he.py:214, in MPL_HyperExplorer.plot.<locals>.plot_sig_and_nav(plot_style)
    212     display(self.signal_plot.figure.canvas)
    213 elif plot_style == "horizontal":
--> 214     display(HBox([self.navigator_plot.figure.canvas,self.signal_plot.figure.canvas]))
    215 else: # plot_style == "vertical":
    216     display(VBox([self.navigator_plot.figure.canvas, self.signal_plot.figure.canvas]))

AttributeError: 'NoneType' object has no attribute 'figure'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b104055) 80.23% compared to head (3dd0912) 80.33%.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           RELEASE_next_patch    #3304      +/-   ##
======================================================
+ Coverage               80.23%   80.33%   +0.10%     
======================================================
  Files                     147      147              
  Lines                   21846    21848       +2     
  Branches                 5140     5141       +1     
======================================================
+ Hits                    17528    17552      +24     
+ Misses                   3112     3083      -29     
- Partials                 1206     1213       +7     

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