Prevent timing issue and set the param ImsCallSessionImplBase to null when calling callSessionMergeStarted.

In some implementations,
1. If the CTS TC waits about 500 ms, the invokeCallSessionMergeFailed is processed well before calling conferenceCall#disconnect.
2. callSessionMergeStarted may not be called if there is an existing conference call session. For this , set the param ImsCallSessionImplBase to null when calling callSessionMergeStarted.

Bug: 297500359
Test: atest ImsCallingTest#testCallJoinExistingConferenceCallAfterCallSwapFail, testConferenceCallFailure

Change-Id: Ia20814d667e058e2a8fb8854bc52f50452ad9b2e