Cherry-pick #18865 to 7.x: index_pattern under visState.params cannot be empty ""

[kaiyan-sheng]

Cherry-pick of PR #18865 to 7.x 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]

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Build stats

• Build Cause: [Started by user kaiyan-sheng, Replayed #1]

• Start Time: 2020-06-04T17:34:39.206+0000

• Duration: 80 min 1 sec

Test stats 🧪

Test	Results
Failed	0
Passed	6665
Skipped	1069
Total	7734