[Alarm] Initialize the "period" to null if its value is not set.

Explicitly set null to the property "period" if period input value is not set. This can ensure the property "period" is always enumerable.

BUG=XWALK-2079
Yes, the "period" is readonly, according to its spec. It's initialized at the Alarm constructor.
I've updated the commit log, thank you for reminding.