Disallow binding to an empty command

This was introduced (most likely accidentally) in
9cbacf3264.

Fixes #3631

doc/changelog.asciidoc

@@ -134,6 +134,7 @@ Fixed
 - HTML/JS resource files are now read into RAM on start to avoid crashes when
   changing qutebrowser versions while it's open.
 - Setting `bindings.key_mappings` to an empty value is now allowed.
+- Binding to an empty command now shows an error rather than crashing.
 
 Removed
 ~~~~~~~

qutebrowser/config/configdata.yml

@@ -2485,9 +2485,7 @@ bindings.commands:
       name: Dict
       none_ok: true
       keytype: Key
-      valtype:
-        name: Command
-        none_ok: true
+      valtype: Command
   desc: >-
     Keybindings mapping keys to commands in different modes.