spatialdb/spatial_db.cc: use !empty() instead of 'size() > 0'

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().

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
ttl/ttl_test.cc: prefer prefix ++operator for non-primitive types

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