feat(_comp_compgen_commands): align return value with other compgens

Co-authored-by: Koichi Murase <myoga.murase@gmail.com>

bash_completion

@@ -2706,7 +2706,7 @@ _complete_as_root()
 # @since 2.12
 _comp_compgen_commands()
 {
-    [[ ! ${cur-} ]] && shopt -q no_empty_cmd_completion && return
+    [[ ! ${cur-} ]] && shopt -q no_empty_cmd_completion && return 1
     # -o filenames for e.g. spaces in paths to and in command names
     _comp_compgen -- -c -o plusdirs && compopt -o filenames
 }