I do think that contexts for to this will vary so greatly that there can't be any one consistent answer. Obviously it depends on existing institutional culture and policies. And also number of students in the class (3, 30, or 300?) and duration of the class (1 hour, 2 or 4?). And level of the class (intro freshmen or graduate seminar?). And proximity of other resources like department or lecturer's office, so as to possibly direct students there (maybe you're at a foreign location where that's not possible). And availability of other work the lecturer can do (e.g., Sascha's answer about work via laptop).
Here's just one example for me: I'm in a pretty traditional math department in the northeast U.S. where almost everyone still lectures with chalk. A few years back when I did the same, I had a trigonometry class during a winter module at 8 AM (think: snowy January, not a residential campus, everyone's on the road, need to leave home before sunrise). Several mornings no one was there at the class start time; and yes, I did start writing the presentation on the board as usual. I figured students could show up and copy what was on the board, which was the only way they could get those notes (and that's what did in fact occur).
Now I've switched my method to use projected digital slides, which are all available for download off the school's learning management system any time at a student's convenience (for which I've had to request room changes outside our usual math department lecture wing). So there isn't the same motivation to mime-out the lecture in students' absence. This semester I had an intro programming course which several times had no students present at the 9 AM start time. In this case I just waited and did some reading until the first student showed up, and commenced a one-on-one conversation about questions or difficulties. Once there were 2 or 3 there I started the normal presentation.
In short: Good responses to this question will vary without bound, depending on context (e.g., even depending on the lecture format itself).