test/ObjectMap/KeyValueDBMemory.cc: use empty() instead of size()

Use empty() instead of 'size() == 0' to fix:

[src/test/ObjectMap/KeyValueDBMemory.cc:83]: (performance)
 Possible inefficient checking for 'db' emptiness.
[src/test/ObjectMap/KeyValueDBMemory.cc:97]: (performance)
 Possible inefficient checking for 'db' emptiness.

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