examples: common connect to also support no connection flow if no inteface chosen

This is useful for testing if there's no need for external network and connection


* Original commit: espressif/esp-idf@085d2b8d25

examples/common_components/protocol_examples_common/include/protocol_examples_common.h

@@ -24,6 +24,11 @@ extern "C" {
 #define EXAMPLE_INTERFACE get_example_netif()
 #endif
 
+#if !defined (CONFIG_EXAMPLE_CONNECT_ETHERNET) && !defined (CONFIG_EXAMPLE_CONNECT_WIFI)
+// This is useful for some tests which do not need a network connection
+#define EXAMPLE_INTERFACE NULL
+#endif
+
 /**
  * @brief Configure Wi-Fi or Ethernet, connect, wait for IP
  *