Fix values of StorageEvent properties.

Fix values of StorageEvent properties.

[Issue#] N/A
[Problem] On removeItem() newValue was set to empty instead of null. On
setItem() oldValue was set to empty instead of null. On clear() key,
oldValue nad newValue were set to empty instead of null.
[Cause] N/A
[Solution] Pass proper values.
[SCMRequest] N/A

[Verification]
1. Build repository
2. Run W3C_WidgetInterface.wgt test widget.
3. Run tests from [RSA]/test/webapi
   - webapi-w3c-widgetinterface-tests/widgetinterface/w3c/au.wgt
   - webapi-w3c-widgetinterface-tests/widgetinterface/w3c/setItem-fires-event.wgt
   - webapi-w3c-widgetinterface-tests/widgetinterface/w3c/removeItem-fires-event.wgt

Change-Id: Ic39340ec283a038012d38dc1918135ea2809e647