Remove inconsistent null value checkings in the ChangeEmail class

Variable ChangeEmail.change is used widely in sending change emails,
sometimes its value is tested to see if it is null, sometimes it is
not tested, we just remove the null value checking for all places,
and let the users of ChangeEmail class to make sure to pass a valid
value.

Change-Id: I26b2d1c2f84e632603b74e5a680b72c922ead3e3
2 files changed