Fix bug in Filters aggregation and Terms aggregation with Other bucket

[wylieconlon]

Summary

The bug is reproduced when you run a Filters aggregation with an empty string in the filter bar, and results in a user-visible toast saying "Can't convert undefined to object". It also threw a console error saying malformed filter agg params, missing "input" query:

The bug is that the empty string was causing the query DSL not to be generated for the default Filters aggregation. This is related to the Terms aggregation with Other bucket, which uses a Filters aggregation to calculate how many documents are not included in the top terms.

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
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For maintainers

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

Pinging @elastic/kibana-app

[wylieconlon]

This is just reformatting

[elasticmachine]

💚 Build Succeeded

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

[Bargs]

LGTM

[TinaHeiligers]

I was too slow!