fix(modem): Fixed UART task to check for buffered data periodically

Closes https://github.com/espressif/esp-protocols/issues/536

components/esp_modem/src/esp_modem_uart.cpp

@@ -160,6 +160,11 @@ void UartTerminal::task()
                 ESP_LOGW(TAG, "unknown uart event type: %d", event.type);
                 break;
             }
+        } else {
+            uart_get_buffered_data_len(uart.port, &len);
+            if (len && on_read) {
+                on_read(nullptr, len);
+            }
         }
     }
 }