[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/data_retention.go

@@ -347,7 +347,7 @@ func searchChannelsInPolicy(c *Context, w http.ResponseWriter, r *http.Request)
 	c.RequirePolicyId()
 	var props *model.ChannelSearch
 	err := json.NewDecoder(r.Body).Decode(&props)
-	if err != nil {
+	if err != nil || props == nil {
 		c.SetInvalidParamWithErr("channel_search", err)
 		return
 	}