worksheet.rows returns an empty tuple
*Created originally on Bitbucket by [Chris Adams](https://bitbucket.org/%7Badba835c-7478-472b-8bc8-a1f548be4840%7D/)*
I noticed a dangerous regression in some code which accidentally upgraded from 2.5.12 to 2.6.0a1:
```
#!python
wb = load_workbook(filename=filename)
for worksheet in wb.worksheets:
rows = worksheet.rows
```
With 2.5, rows will contain the data. With 2.6.0a1 rows will be an empty tuple, so code which attempts to handle empty sheets will silently miss data.
(source https://github.com/LibraryOfCongress/concordia/blob/master/importer/utils/excel.py#L10-L15)
issue