Worth attending discipline's national conference as early grad student if not presenting?
Attending conferences is very useful for several reasons. Usually, the work presented at conferences involve the latest developments in the field. This may provide you with new ideas or tools for your own research. It will give you a good overview of what is happening. You will also become familiar with who is doing what. In addition to this you also have the possibility to get in touch with colleagues and strike up new contacts with persons working on questions for interest to you. This can become useful in the sense that you can possibly start up collaborations or simply become known to others in the field. The latter can be important, for example, when you publish since reviewers might easily be someone you meet during the conference. Obviously, presenting something makes this even easier but being there is far better than not.

The reason I can see from not going is if you do not think the conference is of major interest or if you do not think you will meet people interested in similar questions to yours, in other words if you cannot see any academic benefits from going. Saving money is of course also a valid question if you think you can use that money more wisely on, for example, another conference.

But, in general, go to good conferences regardless of whether you have something to present or not.