Cherry-pick #18865 to 7.8: index_pattern under visState.params cannot be empty ""

[kaiyan-sheng]

Cherry-pick of PR #18865 to 7.8 branch. Original message:

This PR is to add check to make sure index_pattern under visState.params not be empty "". For example in metricbeat dashboard/visualization, when default index is not metricbeat-* in Kibana and index_pattern is set to "", visualizations will take the default index pattern by default. If the default index pattern is not metricbeat-*, then these dashboards will fail and show no data.

This PR is to not only fix the rest of the dashboards having this issue but also adding the check in mage/check.go.

Related PR: #18698
Discuss Forum Link: https://discuss.elastic.co/t/metricbeats-dashboards-for-kubernetes-controller-manager/233402/4

This PR also improved the error message on which visualization failed the test:

>> Dashboard format - module/kubernetes/_meta/kibana/7/dashboard/Metricbeat-kubernetes-controller-manager.json:
   expected index pattern reference 'metricbeat-*': unexpected index pattern reference found in visualization state params: `` in visualization `Controller Process [Metricbeat Kubernetes]`
>> Dashboard format - module/kubernetes/_meta/kibana/7/dashboard/Metricbeat-kubernetes-proxy.json:
   expected index pattern reference 'metricbeat-*': unexpected index pattern reference found in visualization state params: `` in visualization `Proxy CPU [Metricbeat Kubernetes]`
   expected index pattern reference 'metricbeat-*': unexpected index pattern reference found in visualization state params: `` in visualization `Proxy memory [Metricbeat Kubernetes]`

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [jsoriano commented: jenkins run the tests please]

• Start Time: 2020-06-04T14:44:39.648+0000

• Duration: 69 min 52 sec

Test stats 🧪

Test	Results
Failed	0
Passed	6621
Skipped	1053
Total	7674

[jsoriano]

jenkins run the tests please

[jsoriano]

LGTM, but wait for another CI run, there seemed to be problems in the previous one

[kaiyan-sheng]

Merging this one, CI is green 😄