Remove extra null-check on IntervalStats.events.

Now IntervalStats.events field is initialized at construction of
IntervalStats object, we can remove the extra null-check on
IntervalStats.events, make code more clean and readable.

Test: atest UsageStatsDatabaseTest.java
Change-Id: I2580509f41593c1fdf380ba56ebc7a5b6c6f0c5f