fix nullable header

misc/build.func

@@ -300,7 +300,7 @@ get_header() {
     mkdir -p "/usr/local/community-scripts/headers"
 
     # Check if local file already present
-    if [ ! -f "$local_header_path" ]; then
+    if [ ! -s "$local_header_path" ]; then
         curl -fsSL "$header_url" -o "$local_header_path"
         if [ $? -ne 0 ]; then
             echo -e "${WARN}${BOLD}${YLW}Failed to download header for ${app_name}. No header will be displayed.${CL}"