Ensure AAudioStreamBuilder handles null strings

Make sure that a null char* is converted to an empty optional, rather
than crashing due to a bad std::string constructor

Fixes: 182808263
Test: atest android.nativemedia.aaudio.AAudioTests
Change-Id: Ib3ff552b31dcdc52df56775d0d72c1f726b083de
2 files changed