audiopolicy: Fix null port handling in AudioOutputDescriptor

AudioOutputDescriptor can be created with a null port
for a duplicated output descriptor (see
AudioPolicyManager::checkOutputsForDevice). There used to be
a check for null port in getModuleHandle method prior to
ag/Ie90943ee3102cdb8adf89fdd2addd2c279b1e5bf. Restoring
the null check to prevent crash in getModuleHandle.

Bug: 63715585
Change-Id: If3f9563c58cd44ac95c8e27e0560375a404e5db6
Test: make