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
Change-Id: Icebb86bfbecbf2bfce2ca118c5e8ffa21f2db392