DRILL-7306: Disable schema-only batch for new scan framework
The EVF framework is set up to return a "fast schema" empty batch
with only schema as its first batch because, when the code was
written, it seemed that's how we wanted operators to work. However,
DRILL-7305 notes that many operators cannot handle empty batches.
Since the empty-batch bugs show that Drill does not, in fact,
provide a "fast schema" batch, this ticket asks to disable the
feature in the new scan framework. The feature is disabled with
a config option; it can be re-enabled if ever it is needed.
SQL differentiates between two subtle cases, and both are
supported by this change.
1. Empty results: the query found a schema, but no rows
are returned. If no reader returns any rows, but at
least one reader provides a schema, then the scan
returns an empty batch with the schema.
2. Null results: the query found no schema or rows. No
schema is returned. If no reader returns rows or
schema, then the scan returns no batch: it instead
immediately returns a DONE status.
For CSV, an empty file with headers returns the null result set
(because we don't know the schema.) An empty CSV file without headers
returns an empty result set because we do know the schema: it will
always be the columns array.
Old tests validate the original schema-batch mode, new tests
added to validate the no-schema-batch mode.
