test: Remove an unnecessary NULL check in sd-activation

test: Remove an unnecessary NULL check in sd-activation

test_transient_services() was checking whether (config == NULL), then
shortly afterwards dereferencing config->bus_name (which is in fact
dereferenced on all branches of the function). Remove the check and add
some assertions to ensure a Config struct is always passed into the test
(which it currently is).

Coverity ID: 141877

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99999
Reviewed-by: Simon McVittie <smcv@collabora.com>