Check for null metadata on update

Bug: 80281220
Test: runtest bluetooth -c
com.android.bluetooth.avrcp.MediaPlayerWrapperTest

Change-Id: I0751b54fe383e33eeebe06c671aebaa188e6d680
(cherry picked from commit 09b958725cbdb2a19ef0ac667c6e62145141404c)
2 files changed