Don't assume _soong_config_namespaces_key is present

The board configuration doesn't use init_globals(),
which is where _soong_config_namespaces_key normally
is set. Just assume empty dictionaries in
soong_config_namespace where appropriate.

Bug: 201700692
Test: build/bazel/ci/rbc_product_config.sh -b armv8-userdebug
Change-Id: I5f1e52fa0bdde32bff0e8db54861557dcb327190