Adjust invalid time input for 24h format

Now for DISMISS if the time is invalid and  pm/am isn't
 passed in the voice interactor string prints "null". This
  CL changes it so that an empty string is printed so that
   it works better for 24h locales. e.g. if '23:67' is passed in
   nothing is printed for pm/am.

   b/22022605

Change-Id: Iedc5210dbd21ddf595b731d4d502d759fe841208