The method isEmpty() does not exist on TBolier\RethinkQL\Query\...tion\OperationInterface. It seems like you code against a sub-type of TBolier\RethinkQL\Query\...tion\OperationInterface such as TBolier\RethinkQL\Query\TableInterface or TBolier\RethinkQL\Query\...tTransformationCompound.
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation