Question: Sometimes I see the search bar inside already present text "Search ..." and that once clicked is replaced by what the user wants to write:

Which is a better UX design?

a) leave the search bar empty and clean?

b) insert the text "Search ..."?

Answer: 
I would say that having "Search" as a placeholder or having a blank search input amounts to pretty much the same thing.

What I do as a pattern is using that placeholder to help the user by suggesting what to search, as in if you're doing a grocery site then just say "Search for vegetables, meat, fish..." according to what you formulate to be the most obvious search topics.

Also there's a lot of UI's that use the exact same design pattern (icon/input styling) for search inputs and filter inputs so if you have both you might want to use the placeholder to make the difference more obvious.
I guess none of your options are close to how an internet user think about it.

It's just telling you there is a BAR for search, if you need.

I guess an internet user needs a search function while she/he is looking for something.
I guess an computer user knows what is about the processing of search.

So, if a guest doesn't know there is a common feature for searching what they want, then they don't even need to think about what it says.

By the way, have your tried this yet?
[![By the way, have your tried this yet?](https://i.stack.imgur.com/OZbOJ.png)](https://i.stack.imgur.com/OZbOJ.png)