[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/group_test.go

@@ -80,6 +80,10 @@ func TestCreateGroup(t *testing.T) {
 
 	th.App.Srv().SetLicense(model.NewTestLicenseSKU(model.LicenseShortSkuProfessional, "ldap"))
 
+	_, resp, err := th.SystemAdminClient.CreateGroup(context.Background(), nil)
+	require.Error(t, err)
+	CheckBadRequestStatus(t, resp)
+
 	group, _, err := th.SystemAdminClient.CreateGroup(context.Background(), g)
 	require.NoError(t, err)
 
@@ -1582,6 +1586,11 @@ func TestAddMembersToGroup(t *testing.T) {
 
 	th.App.Srv().SetLicense(model.NewTestLicenseSKU(model.LicenseShortSkuProfessional))
 
+	//Empty group members returns bad request
+	_, resp, nullErr := th.SystemAdminClient.UpsertGroupMembers(context.Background(), group.Id, nil)
+	require.Error(t, nullErr)
+	CheckBadRequestStatus(t, resp)
+
 	groupMembers, response, upsertErr := th.SystemAdminClient.UpsertGroupMembers(context.Background(), group.Id, members)
 	require.NoError(t, upsertErr)
 	CheckOKStatus(t, response)
@@ -1654,6 +1663,10 @@ func TestDeleteMembersFromGroup(t *testing.T) {
 
 	th.App.Srv().SetLicense(model.NewTestLicenseSKU(model.LicenseShortSkuProfessional))
 
+	_, resp, nullErr := th.SystemAdminClient.DeleteGroupMembers(context.Background(), group.Id, nil)
+	require.Error(t, nullErr)
+	CheckBadRequestStatus(t, resp)
+
 	groupMembers, response, deleteErr := th.SystemAdminClient.DeleteGroupMembers(context.Background(), group.Id, members)
 	require.NoError(t, deleteErr)
 	CheckOKStatus(t, response)