Revert "[DBClientZabbix] Fix constant value: EVENT_ID_NOT_FOUND"

This reverts commit 85c415b4799ea6b98a7b6d66ab514ddafd83de7a.

Because, in commit mail discussion, there is a conclusion that the
change isn't good.
[DBClientZabbix] Return a EVENT_ID_NOT_FOUND value when max() function
of sqlite3 return NULL.