Do not return null as active device

getActiveDevices can return at most one device for A2DP. Some places
would check just the count, not the value, and treat null as active
device causing crashes later in code.

Bug: 222743223
Test: play music using LE Audio, ensure no crashes in A2DP code
Change-Id: I48d8e43945e75de0a230a61d09dd6e9f3a734772