Added new insights data type: no_type or null_type

This type can be used when there is no data associated with certain keys

Author: vikramdattu

components/esp_diagnostics/include/esp_diagnostics.h

@@ -103,7 +103,8 @@ typedef enum {
     ESP_DIAG_DATA_TYPE_STR,      /*!< Data type string */
     ESP_DIAG_DATA_TYPE_IPv4,     /*!< Data type IPv4 address */
     ESP_DIAG_DATA_TYPE_MAC,      /*!< Data type MAC address */
-    ESP_DIAG_DATA_TYPE_INVALID,  /*!< Not supported by diagnostics */
+    ESP_DIAG_DATA_TYPE_NULL,     /*!< No type */
+    ESP_DIAG_DATA_TYPE_MAX,      /*!< Max type */
 } esp_diag_data_type_t;
 
 /**