Why didn't the bees sting Officer K's hand?

Bees aren't that harmful.

From many apiculture websites, as longs as you're calm and not a threat:

Most danger is avoided by working slowly and carefully and wearing the appropriate protective gear. Also, reacting suddenly to stings around the head entices more bees to sting. Bees dislike dark, woolly, or sweaty materials, so wear clean, light-colored, cotton coveralls or a long-sleeved shirt and full-length pants. Many beekeepers are toughened enough to tolerate a few stings on the hands so they often do not wear gloves, which are slightly cumbersome.

In the same lines:

Once you get used to handling them and realize that none of them wants to sting you (after all, they die if they sting), you will find opening a beehive to be exciting, interesting, and even relaxing. The bees teach you how to act— be slow, calm, careful, and respectful. If you are careful not to crush bees (the primary cause of stinging) and handle them gently and with the proper use of your tools and protective gear, you will only get an occasional sting.

This scene represents many things. First, K is witnessing a bee for the first time. I couldn't find a quote for this, I'm taking this from Paulie_D's comment. He might not know bees can sting him, or he might just be admiring this new creature.

Then, the bees represent the world healing and recovering from the climate holocaust.

How did you decide that he would come across bees?

There are a lot of problems with bees in the world right now. They are disappearing, so the fact that here you can see those creatures still alive and still present, was for me like a little spark of hope in this dystopian universe.

The scene symbolizes how K has a very calm and gentle nature, where he doesn't harm or destroy things he has never seen before. He gently observes and learns from them. This contrasts with the theme of the movie, where the Wallace corporation wants to find the new baby Replicant and dissect him immediately to uncover its secrets.