I would say it depends on the relationship between the asker and the askee.
I have lunch with coworkers regularly and it's pretty casual. So if you have an established friendship with the coworker where you may hang out at in the kitchen and make coffee together and you get on well, then doing something for lunch to talk further about common interests is pretty natural and it would not be necessary to assume that a particular topic is intended.
However, if a coworker asked you to lunch out of the blue with no prior established relationship then it may be safe to assume that there is an intended topic of discussion.
Rule of thumb is: do you know the person well enough to maintain comfortable conversation with them for a significant amount of time (however long lunch is)? Then no topic is implied or needed. Otherwise, you need to have something in mind to talk about, or what would you say?
The personality of the asker and askee may also play a role. A person may generally not make small talk or be too friendly at work, or may wish to separate work from personal discussion. Different people may have different thresholds for what is considered friendly enough for lunch.
So it varies depending on a number of factors. In any case, even if someone else asks about what you wanted to talk about, they probably were just being polite and double checking, in case your expectation of lunch was different from theirs.