handle empty object field

[luhhujbb]

In some indexes mapping, I've some field with this structure:

"myEmptyObjectField": {
"type": "object"
}

Since mapping is dynamic on these indexes, and object is empty elasticsearch cannot guess a type for sub fields, so it has "object" type. (ie empty object).

Because presto row type need at least one field, I just add a condition to drop the field, so that index is queryable.

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[martint]

Thanks for fixing this! Can you add a test to TestElasticsearchIntegrationSmokeTest?

[luhhujbb]

Because of the bad commit signature, I will create a new pull request, with test.