Future-proof available() check to handle negative return values

esphome/components/ld2412/ld2412.cpp

@@ -310,8 +310,10 @@ void LD2412Component::restart_and_read_all_info() {
 }
 
 void LD2412Component::loop() {
+  // All current UART available() implementations return >= 0,
+  // use <= 0 to future-proof against any that may return negative on error.
   int avail = this->available();
-  if (avail == 0) {
+  if (avail <= 0) {
     return;
   }