Drop EmptyScan in favor of allowing empty VirtualTableScan

[benbellick]

VirtualTableScan currently requires at least one row:

substrait-java/core/src/main/java/io/substrait/relation/VirtualTableScan.java

Line 48 in 13309df

assert !rows.isEmpty()

This made sense when the schema was derived from the first row (714eb77), which required EmptyScan as a separate class to represent empty tables. However, since #272, the schema is explicitly provided via .initialSchema() and is independent of the rows. This makes the non-empty constraint outdated and EmptyScan unnecessary.

We should allow empty VirtualTableScan and deprecate EmptyScan.

[benbellick]

Thank you @gord02 for identifying this issue!