fix(script): stop logging an error when Enter accepts the default ACME port

Pressing Enter at the 'Please choose which port to use (default is 80)' prompt left WebPort empty, and bash arithmetic treats an empty string as 0, so the out-of-range branch fired and printed 'Your input is invalid' even though the default was correctly applied. Handle empty input as accepting the default silently, and validate real input with a digits-only regex so non-numeric entries like '8x' get the invalid-input message instead of a bash arithmetic error. Applied to the identical prompt in x-ui.sh, install.sh, and update.sh.

Fixes #5829

install.sh

@@ -679,7 +679,9 @@ ssl_cert_issue() {
     # get the port number for the standalone server
     local WebPort=80
     prompt_or_default WebPort "Please choose which port to use (default is 80): " "80" XUI_ACME_HTTP_PORT
-    if [[ ${WebPort} -gt 65535 || ${WebPort} -lt 1 ]]; then
+    if [[ -z ${WebPort} ]]; then
+        WebPort=80
+    elif [[ ! ${WebPort} =~ ^[1-9][0-9]*$ || ${WebPort} -gt 65535 ]]; then
         echo -e "${yellow}Your input ${WebPort} is invalid, will use default port 80.${plain}"
         WebPort=80
     fi

update.sh

@@ -498,7 +498,9 @@ ssl_cert_issue() {
     # get the port number for the standalone server
     local WebPort=80
     read -rp "Please choose which port to use (default is 80): " WebPort
-    if [[ ${WebPort} -gt 65535 || ${WebPort} -lt 1 ]]; then
+    if [[ -z ${WebPort} ]]; then
+        WebPort=80
+    elif [[ ! ${WebPort} =~ ^[1-9][0-9]*$ || ${WebPort} -gt 65535 ]]; then
         echo -e "${yellow}Your input ${WebPort} is invalid, will use default port 80.${plain}"
         WebPort=80
     fi

x-ui.sh

@@ -1838,7 +1838,9 @@ ssl_cert_issue() {
     # get the port number for the standalone server
     local WebPort=80
     read -rp "Please choose which port to use (default is 80): " WebPort
-    if [[ ${WebPort} -gt 65535 || ${WebPort} -lt 1 ]]; then
+    if [[ -z ${WebPort} ]]; then
+        WebPort=80
+    elif [[ ! ${WebPort} =~ ^[1-9][0-9]*$ || ${WebPort} -gt 65535 ]]; then
         LOGE "Your input ${WebPort} is invalid, will use default port 80."
         WebPort=80
     fi