Operation output check is too restrictive

[wajda]

Causes AbsaOSS/spline-spark-agent#264

The following check constraint in ExecutionPlanPersistentModelBuilder doesn't assume existence of outputs schema with no attributes ([]).

      case (Some(schemaAgnosticTermOp), Some(schemaAwareAnyOp)) =>
        throw new InconsistentEntityException(s"" +
          s"At least one operation defines output schema [#${schemaAwareAnyOp.id}], " +
          s"while some terminal input operations lack of it [#${schemaAgnosticTermOp.id}]")

But there are some specific cases when this can be valid. For instance, when a data frame is created by selecting literals from an empty data frame.

SELECT 42 as x

Technically the execution plan could start from a SELECT operation, but someone would argue that it would not be 100% correct representation of an execution plan. A SELECT operation projects something on something else, so it has to be preceded by another (synthetic) operation with Nil output, and we want to preserve that relation.

[wajda]

The fix requires changing in a Producer model to distinguish between an empty schema and an undefined schema.
Remediation will be implemented in the agent AbsaOSS/spline-spark-agent#264 instead and this issue will be moved to 1.0.0