VirtualTableScan in Substrait is expected to contain both a NamedSchema
(field names in dfs form and types) like any other read rel, plus the
actual data rows. However substrait-java was ignoring the types of the
NamedSchema, instead grabbing type info from the first row of the data.
This however is not always sufficient since the data can contain varying
nullability row-by-row.

This also just simplifies VirtualTableScan and makes it behave more like
one would expect by looking at the Substrait spec as well as other ReadRels