fix NPE in VoipCallMonitor#stopFGSDelegation

Handle null return from account-to-calls map getter to prevent NPE

The code was throwing a NullPointerException when the account-to-calls
map's getter returned null for a given handle, indicating no calls were
associated with that handle.

This commit fixes the issue by adding an explicit null check before
iterating over the set of calls returned by the map. If the set is null,
the loop is skipped, preventing the NullPointerException.

Flag: com.android.server.telecom.flags.voip_call_monitor_refactor
Fixes: 389201983
Test: com.android.server.telecom.tests.VoipCallMonitorTest#
      testStopFgsDelegationWithoutAnyTrackedCalls
Change-Id: I8d6a9d5c2605fa3fcf7df80e1f89068afd553b9a