[SIEM] fix type errors

[angorayc]

Summary

This PR is to fix the type error reported here:
https://elastic.slack.com/archives/CDKUNANRX/p1584706714425300

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser
• This was checked for cross-browser compatibility, including a check against IE11

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)

[patrykkopycinski]

#60736

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 3e949ee

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