MM-23876: Fix for patching channel moderations with a null team schem… (#14239)

* MM-23876: Fix for patching channel moderations with a null team scheme channel guest role.

* MM-23876: Tests the moderations response.

model/role.go

@@ -204,6 +204,10 @@ func PermissionsChangedByPatch(role *Role, patch *RolePatch) []string {
 func ChannelModeratedPermissionsChangedByPatch(role *Role, patch *RolePatch) []string {
 	var result []string
 
+	if role == nil {
+		return result
+	}
+
 	if patch.Permissions == nil {
 		return result
 	}