Fix error thrown on overview page

[ansjcy]

Description

• Fix https://github.com/opensearch-project/query-insights-dashboards/issues/22
• Add unit tests for the overview page to increase the unit test coverage

  query-insights-dashboards/public/pages/TopNQueries             |   92.04 |       82 |     100 |   91.76 |                   
  TopNQueries.tsx                                               |   92.04 |       82 |     100 |   91.76 | 139,163,213-232   

Issues Resolved

https://github.com/opensearch-project/query-insights-dashboards/issues/22

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