Fix for crash while merging 2 audio calls for initiating conference call

Null pointer exception occured while calling callsessionupdated on listener object.
Listener turned null at the time of executor method is on run. Null check is present before setting the call to executor.
Modified the null checks to be inside of the executor.

Test: Conference call in oriole userdebug
Bug: 210701681
Change-Id: Iffeedb669b4abb9b4f32f015aaea4ba3b99c00f5