[Time] Change getDSTTransition return from undefined to null
[Time] Change getDSTTransition return from undefined to null

Based on the tests:

TZDate_getNextDSTTransition.html
TZDate_getPreviousDSTTransition.html

crosswalk-project/tizen-extensions-crosswalk,paritosh-in/tizen-extensions-crosswalk,crosswalk-project/tizen-extensions-crosswalk,pk-sam/tizen-extensions-crosswalk,hgl888/tizen-extensions-crosswalk,hgl888/tizen-extensions-crosswalk,paritosh-in/tizen-extensions-crosswalk,hgl888/tizen-extensions-crosswalk,crosswalk-project/tizen-extensions-crosswalk,paritosh-in/tizen-extensions-crosswalk,crosswalk-project/tizen-extensions-crosswalk,crosswalk-project/tizen-extensions-crosswalk,pk-sam/tizen-extensions-crosswalk,pk-sam/tizen-extensions-crosswalk,hgl888/tizen-extensions-crosswalk,hgl888/tizen-extensions-crosswalk,hgl888/tizen-extensions-crosswalk,pk-sam/tizen-extensions-crosswalk,paritosh-in/tizen-extensions-crosswalk,pk-sam/tizen-extensions-crosswalk,pk-sam/tizen-extensions-crosswalk,crosswalk-project/tizen-extensions-crosswalk,paritosh-in/tizen-extensions-crosswalk,paritosh-in/tizen-extensions-crosswalk