Refactor Node Processing

* Node processing works on the concept of process groups.
* A node group can be inherited, run on main thread, or a sub-thread.
* Groups can be ordered.
* Process priority is now present for physics.

This is the first steps towards implementing https://github.com/godotengine/godot-proposals/issues/6424.
No threading or thread guards exist yet in most of the scene code other than Node. That will have to be added later.

core/templates/hash_set.h

@@ -80,7 +80,7 @@ private:
 	}
 
 	bool _lookup_pos(const TKey &p_key, uint32_t &r_pos) const {
-		if (keys == nullptr) {
+		if (keys == nullptr || num_elements == 0) {
 			return false; // Failed lookups, no elements
 		}
 
@@ -237,7 +237,7 @@ public:
 	}
 
 	void clear() {
-		if (keys == nullptr) {
+		if (keys == nullptr || num_elements == 0) {
 			return;
 		}
 		uint32_t capacity = hash_table_size_primes[capacity_index];