workload/schemachange: return error when a table has no columns

Previously, unexpected syntax errors would occur if the workload
attempted to create an index on a table with no columns.
For example, the workload would generate a statements such as
`CREATE INDEX index2 ON table1 ()`, which are syntactically
invalid. This change updates the function `getTableColumns` to
return an error if a table has no columns to use when creating
an index. The workload will try to generate another operation
when it sees this error.

Release note: None

pkg/workload/schemachange/operation_generator.go

@@ -1629,6 +1629,9 @@ func (og *operationGenerator) getTableColumns(
 	if err := rows.Err(); err != nil {
 		return nil, err
 	}
+	if len(ret) == 0 {
+		return nil, pgx.ErrNoRows
+	}
 	for i := range ret {
 		c := &ret[i]
 		c.typ, err = og.typeFromTypeName(tx, typNames[i])