Don't fail if default namespace isn't found

This fixes a regression caused by 8f4afc82
that libnativeloader can't no longer handle the case that it has failed
to find the default namespace. Previously before the change,
libnativeloader continued to use nullptr as the parent namespace which
is reconized as the default namespace inside the linker.

This change recovers the previous behavior. When the default namespace
is not found, NativeLoaderNamespace object is constructed from nullptr.

Bug: 138607234
Test: run app-compat/app-startup-gce on cf_x86_phone using forrest
Change-Id: If518fbc055399b73e7d3a6b45ace0f71e9c25dae