isthmus: an empty map literal has no ExpressionRexConverter visit, so an empty map column fails to convert

[nielspardon]

ExpressionRexConverter overrides visit(Expression.EmptyListLiteral) but has no override for Expression.EmptyMapLiteral, so an empty map reaches AbstractExpressionVisitor.visitFallback and throws UnsupportedOperationException instead of converting.

NamedStruct schema = NamedStruct.of(List.of("col1", "a"), R.struct(R.map(R.STRING, R.struct(R.I32))));
// row value:
ExpressionCreator.emptyMap(false, R.STRING, R.struct(R.I32))
// -> UnsupportedOperationException: Expression ... not handled by visitor type ...

The empty list case converts correctly through the same paths, so the two container kinds are inconsistently supported.

Found reviewing #1153, which added map handling to the virtual-table value conversion; that code is reachable for a populated or null map but not for an empty one.