Handle case where pyenv-commands --sh returns nothing (#2908)

In exceptional cases (custom installation, malfunctions elsewhere), `pyenv-commands --sh` may return nothing.
In non-Fish, this would cause "syntax error near unexpected token `)'" in `pyenv()`.

Bash does not allow to specify a `case` option that would never match.
This works around it by defaulting to `/`. Commands, being filenames, can never match it.
In Fish, nothing needs to be done: it apparently does interpret a `case` without argument as one that never matches.

libexec/pyenv-init

@@ -297,7 +297,7 @@ EOS
   fi
 
   case "\$command" in
-  ${commands[*]})
+  ${commands[*]:-/})
     eval "\$(pyenv "sh-\$command" "\$@")"
     ;;
   *)