REGR: concat of Sparse with incompatible dtype now gives Sparse[object] instead of object
Regression on master related to the `common_dtype` mechanism, I suppose:

```
In [5]: pd.__version__    
Out[5]: '1.0.3'

In [6]: s1 = pd.Series([1, 0, 2], dtype=pd.SparseDtype("int64", 0)) 

In [7]: s2 = pd.Series(["a", "b", "c"], dtype="category") 

In [8]: pd.concat([s1, s2]) 
Out[8]: 
0    1
1    0
2    2
0    a
1    b
2    c
dtype: object

In [9]: pd.concat([s2, s1]) 
Out[9]: 
0    a
1    b
2    c
0    1
1    0
2    2
dtype: object
```

(and the same on v0.25.3)

But on master:

```
# raising in SparseDtype._get_common_dtype
In [3]: pd.concat([s1, s2])  
...
TypeError: data type not understood

In [4]: pd.concat([s2, s1])   
Out[4]: 
0    a
1    b
2    c
0    1
1    0
2    2
dtype: Sparse[object, 0]
```