[MM-54024] Handle JSON nulls when unmarshalling in api4 (#24656)

* handle JSON nulls when unmarshalling in api4

* catch more cases of unhandled JSON null

* fix lint issues

* remove unnecessary checks

* add bot tests for null values

* fix bot_test lint

* add channel null tests

* add group null tests

* add notify_admin null tests

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/saml.go

@@ -272,7 +272,7 @@ func resetAuthDataToEmail(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 	var params *ResetAuthDataParams
 	jsonErr := json.NewDecoder(r.Body).Decode(&params)
-	if jsonErr != nil {
+	if jsonErr != nil || params == nil {
 		c.Err = model.NewAppError("resetAuthDataToEmail", "model.utils.decode_json.app_error", nil, "", http.StatusBadRequest).Wrap(jsonErr)
 		return
 	}