PERF: readability container size empty

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.

cd
run-clang-tidy.py -extra-arg=-D__clang__ -checks=-*,readability-container-size-empty  -header-filter=.* -fix