cleanup: Use `+` param expansion flag in arithmetic context

src/widgets.zsh

@@ -95,7 +95,7 @@ _zsh_autosuggest_modify() {
 
 # Fetch a new suggestion based on what's currently in the buffer
 _zsh_autosuggest_fetch() {
-	if [[ -n "${ZSH_AUTOSUGGEST_USE_ASYNC+x}" ]]; then
+	if (( ${+ZSH_AUTOSUGGEST_USE_ASYNC} )); then
 		_zsh_autosuggest_async_request "$BUFFER"
 	else
 		local suggestion

zsh-autosuggestions.zsh

@@ -366,7 +366,7 @@ _zsh_autosuggest_modify() {
 
 # Fetch a new suggestion based on what's currently in the buffer
 _zsh_autosuggest_fetch() {
-	if [[ -n "${ZSH_AUTOSUGGEST_USE_ASYNC+x}" ]]; then
+	if (( ${+ZSH_AUTOSUGGEST_USE_ASYNC} )); then
 		_zsh_autosuggest_async_request "$BUFFER"
 	else
 		local suggestion