Can't set `print_area` to None

Example:

from openpyxl import Workbook

wb = Workbook()
ws = wb.active

# Get the current print area, correctly returns None
print(ws.print_area)

# Print area is set correctly
ws.print_area = "A1:B10"
print(ws.print_area)

# Clear the print area
ws.print_area = None # Error here "'NoneType' object is not iterable"
print(ws.print_area)

Can't work out how to get a merge request in but you could fix this by changing line 892 of worksheet.py to:

        if value != None:
            self._print_area = [absolute_coordinate(v) for v in value]
        else:
            self._print_area = None

Test (Line 420 of test_worksheet.py)

    @pytest.mark.parametrize("cell_range, result",
                             [
                                 ("A1:F5",  ["$A$1:$F$5"]),
                                 (["$A$1:$F$5"],  ["$A$1:$F$5"],),
                                 (None, None),
                             ]