MAINT: Safer check

[larsoner]

Every once in a while in testing I get:

________________________________________________________________________ test_plot_instance_components[qt] _________________________________________________________________________
mne/viz/tests/test_ica.py:521: in test_plot_instance_components
    fig._fake_click((x, y), xform="data")
../mne-qt-browser/mne_qt_browser/_pg_figure.py:4665: in _fake_click
    raise RuntimeError(f'There as been an {exc[0]} inside the Qt '
E   RuntimeError: There as been an <class 'RuntimeWarning'> inside the Qt event loop (look above for traceback).
...
Traceback (most recent call last):
  File "/home/larsoner/python/virtualenvs/base/lib/python3.11/site-packages/pyqtgraph/GraphicsScene/GraphicsScene.py", line 378, in sendClickEvent
    item.mouseClickEvent(ev)
  File "/home/larsoner/python/mne-qt-browser/mne_qt_browser/_pg_figure.py", line 612, in mouseClickEvent
    y_values = np.asarray(list(self.ch_texts.values()))[:, 1, :]
               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
IndexError: too many indices for array: array is 1-dimensional, but 3 were indexed

Not quite sure why this happens, but this PR works around the issue by short-circuiting if there are no values.