Add null check before checking overlay

This change prevents an NPE that was occurring when the package
for a given setting was assumed to be non-null while dumping
package information for e.g. adb shell dumpsys package

Test: manual - adb shell dumpsys package is no longer malformed
Change-Id: Idaf6a6b82f1fb450848faa15f19ee08085605bdb
Fixes: 79268525