Better to give a poor/unfinished talk at a conference or cancel it? Here's my dilemma. I met a post-doc at a conference a while back, and told him about the project I was working on at the time, and he subsequently invited me to give a talk in a session at an upcoming (next week) conference.  I accepted, since it was my first invitation to speak at a conference, and overall I was/am excited at this prospect, and at the prospect of collaborating with him in the future. However for various reasons (mostly time commitment on my part), this particular project - which is really a side-project to my main dissertation work - hasn't panned out like I hoped it would when I accepted the talk.  Needless to say, I don't feel confident that my talk will make a good contribution to the session, since my results are very lack-luster and my expertise in other aspects isn't enough to make up for this.  Since I'm a PhD student and this is a side project, I don't have old work to "fall back on" and my advisor is impartial.

So which is better: cancel the talk and apologize to the organizer profusely, perhaps suggesting a future collaboration, or suck it up and give a woefully under prepared talk, risking embarrassment to myself?  I'm leaning towards cancelling, because I feel like a poor talk makes me look bad to everyone who listens, while cancelling only makes me look bad to the session organizer.
When this happens to me, I just describe it as "work in progress" or a "research attempt" and present what I can with what I've got.  What's wrong with that?  That's how research works.

"...hasn't panned out like I hoped it would when I accepted the talk."  You can say this during your talk, and explain why it didn't pan out, and ask the audience if they can help.  Maybe someone in the audience (your next co-author perhaps?) knows how to proceed, and has the expertise you're lacking.  If nobody can help, maybe that's an indication that there is an obstacle (which is important to know).

You've said you're a PhD student, without expertise in the side-topic.  Feel free to mention it in your talk "I'm a PhD student in [some subject] at [some university] under [someone]."

You said a post-doc invited you to give a talk (which I expect is not in the sense of an "invited speaker"), who presumably knows you're an early PhD student, and presumably knows what early PhD student talks are sometimes like.  My suspicion is that this question is more about inexperience and lack of confidence than an inability to give a reasonable talk due to the work being unfinished.