fix(_comp_compgen): clear the variable when no completions are generated

Author: akinomyoga

bash_completion

@@ -863,7 +863,16 @@ if ((BASH_VERSINFO[0] > 5 || BASH_VERSINFO[0] == 5 && BASH_VERSINFO[1] >= 3)); t
         [[ $_dir ]] && command cd -- "$_original_pwd"
 
         ((${#_upvars[@]})) && _comp_unlocal "${_upvars[@]}"
-        ((${#_result[@]})) || return
+
+        # workaround for bash-4.2 nounset
+        ((${#_result[@]})) || {
+            if [[ $_append ]]; then
+                eval -- "$_var+=()"
+            else
+                eval -- "$_var=()"
+            fi
+            return
+        }
 
         if [[ $_prefix ]]; then
             local _i