bugfix ISSUE_801; Remove all comments when only comments

[stropysh]

Fixing issue #801. Format remove all the comments with strip_comments when there are only comments in a line.

Before submitting your pull request please have a look at the
following checklist:

• ran the tests (pytest)
• all style issues addressed (flake8)
• your changes are covered by tests
• your changes are documented, if needed

For example, as it now, if execute this code:

query = "--A;\n--B;"
result = sqlparse.format(query, strip_comments=True)

result is --B

After fix - empy line