Handle null values in AStatsEvent

- Treat NULL strings and byte arrays as zero-length strings/byte arrays.

This maintains backwards compatibility with legacy protocol which
handled nulls.

Bug: 155363739
Test: atest libstatssocket_test
Change-Id: I484b7c968270ae7228ea53bb97c7e6a2dbebe983
Merged-In: I484b7c968270ae7228ea53bb97c7e6a2dbebe983
(cherry picked from commit 7a19241f21ab47b7be70d47dd5297effed900280)
2 files changed