[Index Management] Make fetch indices call more resilient

[alisonelizabeth]

Due to a change in Elasticsearch (elastic/elasticsearch#74687), the call to fetch indices in Index Management is failing, causing the "Indices" tab to error.

While this should be resolved on the ES side, we can make some changes in our code to be more resilient:

1. The problem appears to occur on #L81, as the index is undefined. One improvement here might be to first check that the number of indices is the same as to what is returned from cat indices call and also to verify that the index is defined.
2. Evaluate current API integration tests and add better coverage to cover this scenario.

[elasticmachine]

Pinging @elastic/kibana-stack-management (Team:Stack Management)