ART: Do not JNI abort on nullptr GetObjectRefType

A nullptr is a valid input, as it is different from a null reference.

Bug: 18820997

(cherry picked from commit a87630724ef4f8760684fa69c8ecc685735aff88)

Change-Id: Ibda8907ba13b20d2055049492a356ffdf4ddc714
2 files changed