Null check on permission request check

This change adds a null check on PackageSetting.pkg before passing it on
to the method that checks for the QUERY_ALL_PACKAGES permission on the
package.

Test: atest AppEnumerationTests AppsFilterTest
Bug: 150405193
Change-Id: Ic92e29c685ff215f1c54dbb4aa8a0c6377a30fa7