fix NPE in CallLog.AddCallParametersBuilder#setAssertedDisplayName

It appears some backup & restore CallLog tests are populating setAssertedDisplayName with
a null object. This is causing a NPE to occur.  This XS-CL fixes the NPE.

Fixes: 327754873
Test: existing CTS Coverage
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:30ef1129f8fe75b57f33b193cb9b9b64688263df)
Merged-In: Icebb86bfbecbf2bfce2ca118c5e8ffa21f2db392
Change-Id: Icebb86bfbecbf2bfce2ca118c5e8ffa21f2db392