BUG: Render empty DataFrame as empty HTML table w/o raising IndexError.
 - [X] closes #15953 
 - [X] 1 test added / passed
 - [X] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

A one line change to add an ``if clabels`` guard to clabels enumeration.

Returns HTML string consisting of empty table.

Help requested with format and placement of whatsnew entry.
