Add null checks for ConcurrentHashmap queries

Ensure all queries done in CompatChange are only done for non-null
package names.
The previous change broke several tests.

Bug: 200155796
Test: atest com.android.server.wm.DisplayAreaTest#testGetOrientation
Change-Id: I7fc9ccb45987e03f6f172a077303c25d5ae1ccf3