[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/system.go

@@ -347,7 +347,7 @@ func queryLogs(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	var logFilter *model.LogFilter
 	err := json.NewDecoder(r.Body).Decode(&logFilter)
-	if err != nil {
+	if err != nil || logFilter == nil {
 		c.Err = model.NewAppError("queryLogs", "api.system.logs.invalidFilter", nil, "", http.StatusInternalServerError)
 		return
 	}