'NoneType' object has no attribute 'parent' accessing the Fill property of an empty cell
*Created originally on Bitbucket by [memoselyk (Julian Trevino Mondragon)](https://bitbucket.org/%7Befae3a5a-0ded-4054-84e2-ae0ead0cc61d%7D/)*
**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.
issue