Add null check in the notifyForegroundUserAboutSoundIfNecessary

Bug: 349624618
Test: atest CtsMediaAudioTestCases:android.media.audio.cts.AudioFocusTest#testDuckedUidsAfterMediaMusic -- --abi x86_64 \
Flag: EXEMPT minor bug fix
Change-Id: Ia2b031fde4429f3800ddb9e621ab52316ee2f93c
1 file changed