[Resolver] Remove useless check that breaks when tree has no nodes

[oatkiller]

If process event collection is disabled an alert event can still be triggered that has process.entity_id. Resolver will try to graph it but it will have no process events. The error would be triggered. Without the error, the user gets an empty resolver, but at least it doesn't crash.

Checklist

Delete any items that are not applicable to this PR.

• Unit or functional tests were updated or added to match the most common scenarios

For maintainers

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

[kibanamachine]

💛 Build succeeded, but was flaky

• continuous-integration/kibana-ci/pull-request
• Commit: cd95219
• Flaky suites:
  • xpack-securitySolutionCypress

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Test Failures

Kibana Pipeline / kibana-xpack-agent / persist the deletion of a column.persistent timeline persist the deletion of a column

Link to Jenkins

Stack Trace

AssertionError: Timed out retrying: Too many elements found. Found '9', expected '8'.
    at Context.eval (http://localhost:61141/__cypress/tests?p=cypress/integration/timeline_local_storage.spec.ts:684:49)

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Build metrics

async chunks size

id	value	diff	baseline
securitySolution	7.3MB	-33.0B	7.3MB

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@elasticmachine merge upstream