When GetStatistics is called on an empty database, SQL SUM() returns NULL
which causes a scan error when converting to int. Wrap SUM expressions with
COALESCE to return 0 instead of NULL.
Add TestGetStatistics to verify both empty and populated database cases.
Fixes issue where `bd stats` and MCP stats tool crash on fresh databases.
Signed-off-by: Joshua Shanks <jjshanks@gmail.com>