`pivot_table` with `margins=True` shows `NaN` with `Period` columns
The following code reproduces the issue:

```python
import pandas as pd

df = pd.DataFrame(
    {
        "a": [1, 1, 2, 2],
        "b": [
            pd.Period("2019Q1"),
            pd.Period("2019Q2"),
            pd.Period("2019Q1"),
            pd.Period("2019Q2"),
        ],
        "x": 1.0,
    }
)

df.pivot_table(index="a", columns="b", values="x", margins=True)
```

Output:

```
b 	2019Q1 	2019Q2 	All
a 			
1 	1.0 	1.0 	1.0
2 	1.0 	1.0 	1.0
All	NaN 	NaN 	1.0
```
Expected output:

```
b 	2019Q1 	2019Q2 	All
a 			
1 	1.0 	1.0 	1.0
2 	1.0 	1.0 	1.0
All	1.0 	1.0 	1.0
```