pg2arrow: moved empty results check to the end of file creation.

issue reported at #824.
the old implementation checked empty results per worker basis.

arrow-tools/pgsql_client.c

@@ -1059,8 +1059,6 @@ sqldb_begin_query(void *sqldb_state,
 	pgstate->res = res;
 	pgstate->nitems = PQntuples(res);
 	pgstate->index  = 0;
-	if (pgstate->nitems == 0)
-		Elog("SQL query has empty results: %s", sqldb_command);
 
 	for (int depth=1; depth <= pgstate->n_depth; depth++)
 	{

arrow-tools/sql2arrow.c

@@ -1604,6 +1604,14 @@ int main(int argc, char * const argv[])
 										0);
 		sql_table_clear(table);
 	}
+
+	/* check whether any results were written */
+	if (table->numRecordBatches == 0)
+	{
+		if (!append_filename)
+			unlink(table->filename);
+		Elog("SQL query has empty results: %s", sqldb_command);
+	}
 	/* write out footer portion */
 	writeArrowFooter(table);