[Performance improvement] Duplicate empty node check removal from Subscription.hpp

[PavloZMN]

Describe problem solved by the proposed feature

In class Subscription.hpp, check for empty field _node can be called two or three times in a row.
For example, method updated():

	bool updated()
	{
		if (!valid()) {
			subscribe();
		}

		return valid() ? Manager::updates_available(_node, _last_generation) : false;
	}

Method valid contains check for empty _node:
bool valid() const { return _node != nullptr; }

Method subscribe has the same check:

	bool Subscription::subscribe()
{
	if (_node != nullptr) {
		return true;
	}
       ...

Means, check _node != nullptr can be called two or three times in a row.

Describe your preferred solution

Only subscribe method is used for _node emptiness check.
Performance and code readability will be improved.

Describe possible alternatives

Current version is working fine.

Additional context

No response