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

(cherry picked from commit f65fde73dab73f9ba3c9f856785db251134ef1cc)

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 += ",";