Since metricbeat light modules support processors (elastic#15923), module
initialization requires a publisher in the beat so modules can attach
their processors. `metricbeat test modules` is not initializing as
normal metricbeat commands, and it is not initializing any output or
publisher pipeline, so metricbeat panics when trying to initialize
modules with the new method.

This change adds a dummy publisher for this case, and fixes also a
condition that was adding a `nil` module option, causing additional
panics. A test that reproduced the issues is also added.

(cherry picked from commit 25b8bf1)