QST: | (or) behavior when working with two series that have different indexes
- [x] I have searched the [[pandas] tag](https://stackoverflow.com/questions/tagged/pandas) on StackOverflow for similar questions.

- [x] I have asked my usage related question on [StackOverflow](https://stackoverflow.com).

---

#### Question about pandas

When using the | operation between two dataframes, the order of the operation can give different results if you have nan, or different index. 

```python
df_1 = pd.DataFrame([None], index=[1])
df_2 = pd.DataFrame([True], index=[1])

print(df_1 | df_2)
print(df_2 | df_1)
```
will give you different results: 

```python
       0
1  False
      0
1  True
```
The same things with different index:

```python
df_1 = pd.DataFrame([True], index=[1])
df_2 = pd.DataFrame([True], index=[2])

print(df_1 | df_2)
print(df_2 | df_1)
```
will return 

```python
       0
1   True
2  False
       0
1  False
2   True
```

Is this a normal behavior or a bug? If it is a normal behavior, what is the logic behind it? 

Thanks!