feat: add empty message type to monitor for timeouts

packages/atclient/include/atclient/monitor.h

@@ -19,13 +19,15 @@ enum atclient_monitor_response_type {
   ATCLIENT_MONITOR_MESSAGE_TYPE_NOTIFICATION,   // indicates caller to access `notification` from the union
   ATCLIENT_MONITOR_MESSAGE_TYPE_DATA_RESPONSE,  // indicates caller to access `data_response` from the union
   ATCLIENT_MONITOR_MESSAGE_TYPE_ERROR_RESPONSE, // indicates caller to access `error_response` from the union
+  ATCLIENT_MONITOR_MESSAGE_TYPE_EMPTY,          // indicates that no message was received
 
   // the following 3 enums help indicate what type of error occurred when reading from the monitor connection, you will
   // expect one of these enums along with a non-zero return value from atclient_monitor_read
   ATCLIENT_MONITOR_ERROR_READ, // could be a read timeout or some other error, indicates the caller to access
                                // `error_read` from the union
   ATCLIENT_MONITOR_ERROR_PARSE_NOTIFICATION,
   ATCLIENT_MONITOR_ERROR_DECRYPT_NOTIFICATION,
+
 };
 
 // Represents error information when `ATCLIENT_MONITOR_ERROR_READ` is the message type given by atclient_monitor_read

packages/atclient/src/monitor.c

@@ -152,7 +152,10 @@ int atclient_monitor_read(atclient *monitor_conn, atclient *atclient, atclient_m
       case MBEDTLS_ERR_SSL_WANT_WRITE:         // handshake incomplete
         usleep(10000);                         // Try again in 10 milliseconds
         break;
-
+        // Timeout means nothing to read, return EMPTY message type
+      case MBEDTLS_ERR_SSL_TIMEOUT:
+        message->type = ATCLIENT_MONITOR_MESSAGE_TYPE_EMPTY;
+        return 0;
         // Monitor connection bad, must be discarded
       case MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY: // transport closed with close notify
       case 0:                                 // transport closed without close notify