No traders in diamond city market

Is there just no NPCs around at all? Or is it just the vendors that are missing?

Is it night time? Shops are only open 8am - 8pm -- you can check the in-game time at the bottom of the data tab in the pipboy menu.

No it's not night

Are the guards still there walking around?

And the vendors are just totally missing? Has any combat taken place there, they've not been killed?

I just did a quick google search and did find someone else mentioning something similar -- have you done the quest "Story of the Century" where you speak to Piper in her office? This person said the vendors shown back up after that.

the ai behavior is sometimes like an actor running late for their performance on stage. try sleeping/waiting through a few npc cycles, hopefully that gives them the time to get into their respective places.