Cherry-pick #18797 to 7.8: Fix panic on metricbeat test modules

[jsoriano]

Cherry-pick of PR #18797 to 7.8 branch. Original message:

What does this PR do?

Fix panic on `metricbeat test modules`

Since metricbeat light modules support processors (#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 issue is also added.

Why is it important?

metricbeat test modules is not working with modules defined in the main configuration file.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

• Check that the steps described in metricbeat test modules panics #18789 don't cause a panic.

Related issues

• Related to [Metricbeat] Support processors defined for light modules #15923
• Fixes metricbeat test modules panics #18789

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #18852 updated]

• Start Time: 2020-06-03T13:47:33.714+0000

• Duration: 83 min 3 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2833
Skipped	615
Total	3448

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[ChrsMark]

ah, it seems there is a conflict in changelog @jsoriano ?