ART: Sometimes even empty methods take forever to verify

In cases of very high load and/or bad scheduling, the verifier may
take longer than the threshold duration to verify an empty method.
The LargeMethod detection needs to accept that the code_item may
be null.

Bug: 21364300

(cherry picked from commit 81104473)

Change-Id: Iceff3e4688cc1a5fe7a836f7a9bf6c49a392b618