Pipeline test config panics if fields is empty

[jsoriano]

Parsing pipeline test config panics if the fields attribute is empty. This can be reproduced for example by trying to skip a test with something like the following:

skip:
  reason: New value in network.community_id when running on newer versions
  link: https://github.com/elastic/integrations/issues/10904

Configuring it like this works:

skip:
  reason: New value in network.community_id when running on newer versions
  link: https://github.com/elastic/integrations/issues/10904
fields: 
  tags: []

It cannot be reproduced configuring the common test config only. Maybe the issue is related to merging the common test config and the config specific to the test.

Seen in elastic/integrations#10919, trying to skip pipeline tests in the cisco_ios package.