[Task Manager] Fixes error when we claim new tasks beyond capacity

[gmmorris]

Fixed issue where we would try and claim new tasks even when there are no available workers

Summary

An error was introduced recently when Task Manager tries to claim tasks despite having no available workers to do so, resulting in an invalid updateByQuery being submitted on every pool refresh interval, flooding the logs.

Checklist

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• 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/kibana-stack-services (Team:Stack Services)

[Bamieh]

Code LGTM. Haven't pulled the PR locally

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: 7f70c55

[bmcconaghy]

Code LGTM

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: d1e1fc9