Replace size method call of containers to empty method where appropriate
This patch was generated by a clang tidy checker that is being open sourced.
The documentation of that checker is the following:
/ The emptiness of a container should be checked using the empty method
/ instead of the size method. It is not guaranteed that size is a
/ constant-time function, and it is generally more efficient and also shows
/ clearer intent to use empty. Furthermore some containers may implement the
/ empty method but not implement the size method. Using empty whenever
/ possible makes it easier to switch to another container in the future.
Patch by Gábor Horváth!