Fix potential NPE in TelephonyEventLog.

I ran into a case in VT testing where the session.getCallId() was
throwing an NPE.
This code cleans up that potential source of an NPE and also protects
against the fact that a call ID in an ImsCallSession is a string and
could technically not contain a number.
Better than crashing telephony.

Bug: 26909254
Change-Id: Id6fe0b46c6c2b46fac29b2e04384bd69e8944373