metrics: don't crash if `address` is NULL

We check earlier if `address != nullptr` in this function; it stands to
reason that we should have the same check here, as well.

Potential use of null was caught by the static analyzer:
> system/bt/gd/common/metrics.cc:74:9: warning: Called C++ object
pointer is null [clang-analyzer-core.CallAndMessage]

Tag: #security
Bug: 181819141
Test: TreeHugger
Change-Id: I4baf5fe648c8fd841875515b024e1658b7ea20cd