Don't fail catastrophically on non-existing namespace

When a new namespace is added to ld.config.txt, it has to be reflected
to native_loader_test.cpp. However, failing to do so crashes the test
instead of showing an error message that the namespace hasn't been
added.

Fixing the issue by not returning nullptr when the namespace isn't
found.

Bug: 138304345
Test: libnativeloader_test
Change-Id: I9eee1293956caac69f95a946604a2e6f665bde89