pd.concat(objs) drops type of the inputs if all inputs are empty
When concatenating several pd.Series of the same type, I would expect the output Series to have the same type. However, this does not hold if all input Series are empty:

```
In [0]: x = pd.Series([], dtype=int)
In [1]: pd.concat([x, x])
Series([], dtype: object)
```

How I would expect it to work:
_pd.concat([x, x]) should return Series([], dtype=int)_

Just as an example, this works fine:

```
In [2]: y = pd.Series([1], dtype=int)
In [3]: pd.concat([y, y])  # This works fine:
0    1
0    1
dtype: int64
```
