[SPARK-13616][SQL] Let SQLBuilder convert logical plan without a project on top of it

JIRA: https://issues.apache.org/jira/browse/SPARK-13616

## What changes were proposed in this pull request?

It is possibly that a logical plan has been removed `Project` from the top of it. Or the plan doesn't has a top `Project` from the beginning because it is not necessary. Currently the `SQLBuilder` can't convert such plans back to SQL. This change is to add this feature.

## How was this patch tested?

A test is added to `LogicalPlanToSQLSuite`.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #11466 from viirya/sqlbuilder-notopselect.