'NoneType' object has no attribute 'parent' accessing the Fill property of an empty cell

Created originally on Bitbucket by memoselyk (Julian Trevino Mondragon)

Preconditions

Create an empty workbook and add only content to cell B2.

Steps to reproduce

1. Open the workbook from precondition
2. Attempt to get the fill property of a cell obtained by iterating over rows from workbook.rows.

Sample snippet showing the failure,

#!python
from openpyxl import load_workbook

wb = load_workbook("test_file.xlsx", read_only=True)
ws = wb.get_active_sheet()

for ri, row in enumerate(ws.rows):
    for rc, cell in enumerate(row):
        col = chr(rc + ord('A'))
        try:
            print "%s, %d - %s" % (col, ri + 1, cell.fill)
        except Exception as err:
            print "%s, %d - ERROR %s" % (col, ri + 1, err)

Observed results

Attemps to obtain the fill properties of cells A1, A2, B1 faill with error:

  File "C:\Python27\lib\site-packages\openpyxl\cell\read_only.py", line 82, in fill
    _id = self.style_array.fillId
  File "C:\Python27\lib\site-packages\openpyxl\cell\read_only.py", line 65, in style_array
    return self.parent.parent._cell_styles[self._style_id]
AttributeError: 'NoneType' object has no attribute 'parent'

Expected results

I presume that such cells should, at least, return a None value instead of raising an exception.