Error on empty date cell
*Created originally on Bitbucket by [Adam Morris](https://bitbucket.org/%7Bb1c90e89-df41-4f4c-a8f9-4f397983fa8b%7D/)*
Excel allows setting the number format of an empty cell. If the cell is set to the date format, the value is None, and the attempt to read the value raises a Type Error, as it can't add the calendar offset to None.
Not sure I understand the from_excel() function in date_time.__init__ fully - would it be an acceptable patch just to return None in this case if the value is None?
```
#!python
from openpyxl import load_workbook
wb = load_workbook("test_date.xlsx")
ws = wb.active
ws.cell('A1').value
# datetime.datetime(2014, 1, 31, 0, 0)
ws.cell('A2').value
## Returns Error:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "openpyxl/cell/cell.py", line 347, in value
value = from_excel(value, self.base_date)
File "openpyxl/compat/functools.py", line 122, in wrapper
result = user_function(*args, **kwds)
File "openpyxl/date_time/__init__.py", line 54, in from_excel
parts = list(jd2gcal(MJD_0, value + offset - MJD_0))
TypeError: unsupported operand type(s) for +: 'NoneType' and 'float'
```
*Attachments:* [test_date.xlsx](/uploads/8fa2208b47b230172ac9db4d6b211e4d/test_date.xlsx)
issue