Avoid NPE crash system server

The install request was removed from the array but another new post
to handle the same request was coming. Cannot get the request from
the array that caused NPE. There are two paths that will post
POST_INSTALL, one comes install flow, one comes from another system
services, e.g, backup, rollback. From dashboard, this should be a
rare case, the happen rate is low. To avoid crash system, do
nothing if request is null.

Bug: 321267572
Test: manual. Cannot reproduce locally
Change-Id: I3ab158a14823719478de2a20c11941ae75b6f549