[SIEM] Fixes a crash when Machine Learning influencers is an undefined value

[FrankHassanabad]

Summary

Fixes a crashing bug when Machine Learning influencers is an undefined value:

https://github.com/elastic/ingest-dev/issues/611

TypeError: Cannot read property 'find' of undefined
    at getNetworkFromInfluencers (https://siem-dev-kibana.app.elstc.co/bundles/siem.bundle.js:41744:33)
    at https://siem-dev-kibana.app.elstc.co/bundles/siem.bundle.js:43012:83
    at Array.reduce (<anonymous>)
    at convertAnomaliesToNetwork (https://siem-dev-kibana.app.elstc.co/bundles/siem.bundle.js:43004:32)
    at https://siem-dev-kibana.app.elstc.co/bundles/siem.bundle.js:42830:78
    at renderWithHooks (webpack://%5Bname%5D/./node_modules/react-dom/cjs/react-dom.development.js?:12839:18)
    at updateFunctionComponent (webpack://%5Bname%5D/./node_modules/react-dom/cjs/react-dom.development.js?:14421:20)
    at beginWork (webpack://%5Bname%5D/./node_modules/react-dom/cjs/react-dom.development.js?:15431:16)
    at performUnitOfWork (webpack://%5Bname%5D/./node_modules/react-dom/cjs/react-dom.development.js?:19106:12)
    at workLoop (webpack://%5Bname%5D/./node_modules/react-dom/cjs/react-dom.development.js?:19146:24)

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

- [ ] This was checked for cross-browser compatibility, including a check against IE11

- [ ] 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

For maintainers

- [ ] This was checked for breaking API changes and was labeled appropriately

• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/secops

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[spong]

Nice clean fix with the [] default value here and within getHostNameFromInfluencers / getNetworkFromInfluencers . This'll still be susceptible to null it looks like, but shouldn't be a problem as influencers should be either undefined or [] coming back from the ML API call.

[spong]

Checked out, verified fix on siem-dev using the crashy-mc-crashes ML Job, and performed a code review. Thanks for adding thorough test coverage with this fix as well -- LGTM 👍