Fix panic when receiving a config to update to if some settings are not set (#13889)

api4/config_test.go

@@ -103,6 +103,11 @@ func TestUpdateConfig(t *testing.T) {
 
 	require.Equal(t, SiteName, cfg.TeamSettings.SiteName, "It should update the SiteName")
 
+	t.Run("Should set defaults for missing fields", func(t *testing.T) {
+		_, appErr := th.SystemAdminClient.DoApiPut(th.SystemAdminClient.GetConfigRoute(), `{"ServiceSettings":{}}`)
+		require.Nil(t, appErr)
+	})
+
 	t.Run("Should fail with validation error if invalid config setting is passed", func(t *testing.T) {
 		//Revert the change
 		badcfg := cfg.Clone()