Make dumpHexString() tolerate null arrays.

Rationale: hexdumps are mainly used when verbose logging is enabled,
which means that callers are rarely exercised (let alone tested).
Crashing on unchecked null pointers doesn't make debugging any easier,
nor production code any more robust.

Moreover, this is the behavior of system.out.println() and other
logging APIs.

Test: runtest -x core/tests/coretests/src/com/android/internal/util/HexDumpTest.java
Bug: 110177912
Change-Id: Idccd81a5654ed0f7fee6b27177941bf8c311973e