Suppress null-dereference warning temporarily

There's one more occurance of this warning.

In file included from hardware/google/av/codec2/tests/C2SampleComponent_test.cpp:23:
In file included from frameworks/av/media/codec2/core/include/C2Component.h:29:
In file included from frameworks/av/media/codec2/core/include/C2Enum.h:20:
frameworks/av/media/codec2/core/include/C2Param.h:1612:92: error: binding dereferenced null pointer to reference has undefined behavior [-Werror,-Wnull-dereference]
C2FieldDescriptor::NamedValuesType named = C2FieldDescriptor::namedValuesFor(*(T*)0);
^~~~~~

Also modernised the code to use nullptr instead of 0.

Bug: 122154294
Test: m checkbuild
Exempt-From-Owner-Approval: No functionality change, similar CL already reviewed by owner
Change-Id: Ieec3335d2f52f0c6f07be1e7c99e4f36c882dabf