Add default styles.xml to make date columns work.

I had to add a `styles.xml` file to be able to use date columns, otherwise excel would complain about a broken file when trying to open it.

In particular the `<cellXfs>` tag seems to be necessary:

```xml
  <cellXfs count="2">
    <xf numFmtId="0" fontId="0" fillId="0" borderId="0" xfId="0" />
    <xf numFmtId="14" fontId="0" fillId="0" borderId="0" xfId="0" />
  </cellXfs>
```

because date cells are written as

```xml
<x:c r="A2" s="1" t="n"><x:v>41791</x:v></x:c>
```

where `s="1"` references the second cell style which makes excel complain when this style is not defined in a `styles.xml`.

I testet this with Excel for Mac 14.4.6 (141106).