2.5.x load_workbook Errors on PivotTable Style When Style Has No Name
*Created originally on Bitbucket by [bmritz (Brian Ritz)](https://bitbucket.org/%7B6b330916-d7fe-4efc-b061-7067ac59515b%7D/)*
Not all Pivot Table styles have the name attribute required by the class `PivotTableStyle` in openpyxl / openpyxl / pivot / table.py. One such style is the "Plain" style of pivottable that is the very first choice on the far left when choosing styles in design tab on mac.
When I added `allow_none=True` on the name attribute, the workbook loaded successfully.
I've attached a toy example of a workbook that has a plain style that throws the following error on load_workbook:
```
In [104]: d = load_workbook("/data/pivottablewithstyle.xlsx")
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-104-9bb3e4269703> in <module>()
----> 1 d = load_workbook("/data/pivottablewithstyle.xlsx")
/usr/local/lib/python3.6/site-packages/openpyxl/reader/excel.py in load_workbook(filename, read_only, keep_vba, data_only, guess_types, keep_links)
280 src = archive.read(pivot_path)
281 tree = fromstring(src)
--> 282 pivot = TableDefinition.from_tree(tree)
283 pivot.cache = pivot_caches[pivot.cacheId]
284 ws.add_pivot(pivot)
/usr/local/lib/python3.6/site-packages/openpyxl/descriptors/serialisable.py in from_tree(cls, node)
81 if hasattr(desc.expected_type, "from_tree"):
82 #complex type
---> 83 obj = desc.expected_type.from_tree(el)
84 else:
85 #primitive
/usr/local/lib/python3.6/site-packages/openpyxl/descriptors/serialisable.py in from_tree(cls, node)
97 attrib[tag] = obj
98
---> 99 return cls(**attrib)
100
101
/usr/local/lib/python3.6/site-packages/openpyxl/pivot/table.py in __init__(self, name, showRowHeaders, showColHeaders, showRowStripes, showColStripes, showLastColumn)
190 showLastColumn=None,
191 ):
--> 192 self.name = name
193 self.showRowHeaders = showRowHeaders
194 self.showColHeaders = showColHeaders
/usr/local/lib/python3.6/site-packages/openpyxl/descriptors/base.py in __set__(self, instance, value)
42 if (not self.allow_none
43 or (self.allow_none and value is not None)):
---> 44 raise TypeError('expected ' + str(self.expected_type))
45 super(Typed, self).__set__(instance, value)
46
TypeError: expected <class 'str'>
```
*Attachments:* [pivottablewithstyle.xlsx](/uploads/1bd6cb0cd597ca5362ebc090260fd721/pivottablewithstyle.xlsx)
issue