Optimizing `TracingAgent::start()`

I believe that the DispatchResponse TracingAgent::start() function can be improved to be more efficient and more readable for those who wish to contribute.

1. Unified the two error checking conditions into one;
2. Used the length() function to check whether at least one category has been enabled;
3. Used the set initialization std::set to insert elements directly from an array;
4. Unified the two null checks into one.

I believe that this way, it is easier to interpret what is happening in this snippet code and look for further improvements.