[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/channel_local.go

@@ -40,7 +40,7 @@ func (api *API) InitChannelLocal() {
 func localCreateChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 	var channel *model.Channel
 	err := json.NewDecoder(r.Body).Decode(&channel)
-	if err != nil {
+	if err != nil || channel == nil {
 		c.SetInvalidParamWithErr("channel", err)
 		return
 	}
@@ -288,7 +288,7 @@ func localPatchChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	var patch *model.ChannelPatch
 	err := json.NewDecoder(r.Body).Decode(&patch)
-	if err != nil {
+	if err != nil || patch == nil {
 		c.SetInvalidParamWithErr("channel", err)
 		return
 	}