Handle empty keys gracefully

[mbrenig]

Traceback (most recent call last):
File "C:/Python27/Scripts/test.py", line 46, in
target.sync(group_tracking_data)
File "C:\Python27\lib\site-packages\sheetsync__init__.py", line 864, in sync
return self.update(raw_data, row_change_callback, delete_rows=True)
File "C:\Python27\lib\site-packages\sheetsync__init_.py", line 933, in update
self.delete_flag_row(key_tuple, wks_row)
File "C:\Python27\lib\site-packages\sheetsync__init.py", line 996, in _delete_flag_row
cell_elem.cell.inputValue = cell_elem.cell.text+DELETE_ME_FLAG
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'