Added No bone set state in the IK

The problem is that initially the root bone was not set, and you didn't know that because the "no set" state was missing. Now I've added it. https://github.com/godotengine/godot-docs/issues/2333

scene/animation/skeleton_ik.cpp

@@ -335,7 +335,7 @@ void SkeletonIK::_validate_property(PropertyInfo &property) const {
 
 		if (skeleton) {
 
-			String names;
+			String names("--,");
 			for (int i = 0; i < skeleton->get_bone_count(); i++) {
 				if (i > 0)
 					names += ",";