Use default icon if package name is null

SystemUI crashes when package name is not provided due to changes made in ag/24363286. There is no need to use checkNotNull as we do show default icon when package name is null.

Flag: NONE
Test: atest MediaTttSenderCoordinatorTest
Test: adb shell cmd statusbar media-ttt-chip-sender MyTablet ALMOST_CLOSE_TO_START_CAST useAppIcon=false
Bug: 215562505
Change-Id: I054c0a9930b8d21d018a8afe2c03abe8dda93c1e