Dataless colorbars break when updated #13963

#13171 documented how to construct a colorbar "without an associated artist", via fig.colorbar(cm.ScalarMappable(norm=norm, cmap=cmap), ax=ax). It is also possible e.g. to update the colorbar via

sm = ScalarMappable(norm=norm, cmap=cmap)
fig.colorbar(sm)
<...>
sm.set_cmap("plasma")

(Although this is only documented in master, the recipe has existed for a while.)

But this got broken by #13234: one now gets

Traceback (most recent call last):
  File "lib/matplotlib/cbook/__init__.py", line 214, in process
    func(*args, **kwargs)
  File "lib/matplotlib/colorbar.py", line 1189, in on_mappable_changed
    self.update_normal(mappable)
  File "lib/matplotlib/colorbar.py", line 1230, in update_normal
    self.set_alpha(mappable.get_alpha())
AttributeError: 'ScalarMappable' object has no attribute 'get_alpha'

Indeed, the ScalarMappable class does not define get_alpha(). I think the solutions are either to make Colorbar.update_normal first check for whether such a method exists, or to add a dummy get_alpha() (always returning 1) to ScalarMappable.

Allowing myself to label as release critical (as a regression) but feel free to relabel if you disagree.

mpl master, arch linux, py3.7