Converting an empty list to relational algebra throws

[julianhyde]

In hybrid mode (where we convert Morel to expressions to relational algebra) a query on an empty list gives an error:

= Sys.set ("hybrid", true);
val it = () : unit
= from e in [] yield {a = 1, b = 2};
bad INTERSECT/UNION/EXCEPT input count

The problem is that we are trying to create a Calcite Union relational operator with zero inputs.

[julianhyde]

Fixed in 78d3c33.