No added value
Can you relate to or have you ever experienced a similar "conversation":
A: I can see nothing demonstrable in the current sprint task list.
B: I beg your pardon? It's the second to last sprint before the release and there's a ton of work still not finished.
Yes, there are people who, for any reason, dislike doing demos (I am a member of the club myself). And certainly all projects have sprints in which the majority of work was solely refactoring, bug fixing, testing, and so on.
Although, claiming there is nothing to showcase while the release date is approaching fast and the MVP is still incomplete clearly isn't right.
Demo fear is ever present
The topic popped up into the discussion, as you've might expected, over a glass of beer. Every single developer in our small group encountered demo fear to some extent. We have all worked for multiple companies and are not coworkers so the tales source is rich. Each of us either dislike doing demos or has/had a colleague who doesn't like it.
We have all agreed the worst anti demo "motive" is being unable to spot value. Of course, showcasing command line interfaces, database query speed up, a bunch of REST endpoints or their improvements isn't the most interesting demo imaginable. But it was created and exists for a reason and therefore is valuable.
Or can the sentence "there's nothing to demo" be translated as "I haven't done anything useful"? And if so, what is the story behind? You see, crime should be punished, not failure. Why have I failed? Is it because I got stuck, the story wasn't refined enough, the estimate was way off, I didn't know what to do or how to solve the problem at hand?
It's all about communication
The fact that you consider boring what you have worked on in the last sprint most probably isn't congruent with your stakeholders opinion. When a REST interface provides data to a pleasant user facing JS app, it is implicitly demoed together with the UI. So, please, with cream and sugar on top, ask the front-end developer to mention what is going on under the hood.
And in case of the less favorable situation, try to ask yourself what really happened. Have I communicated my blockers? What was I reporting during the daily stand-up? Did anyone (scrum master, other team members, stakeholders) have a clue what was going on? Was the story flagged as potentially problematic? Failing to deliver value is OK (well, unless it's frequent). If anything, it points out places for improvement and is a great candidate for the following retrospective meeting.
I've been where your are. Change your mindset and focus on added/sliced value during the next demo prep meeting. Please keep those "there's nothing interested" elusions to yourself.