src/client/SyntheticClient.cc: use !subdirs.empty() instead of size()

Use empty() since it should be prefered as it has, following the
standard, a constant time complexity regardless of the containter
type. The same is not guaranteed for size().

warning from cppchecker was:
[src/client/SyntheticClient.cc:2706]: (performance) Possible
  inefficient checking for 'subdirs' emptiness

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>