bug#54044: 27.2; `describe-function': Don't show empty list of key bindings

[Drew Adams]

emacs -Q
C-h f superword-mode

The *Help* output includes this, at the end:


key             binding
---             -------


Please consider not showing such an empty list of bindings.

In GNU Emacs 27.2 (build 1, x86_64-w64-mingw32)
 of 2021-03-26 built on CIRROCUMULUS
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.19043
System Description: Microsoft Windows 10 Pro (v10.0.2009.19043.1466)