BUG: Series.fillna is altering None values of unspecified columns
- [x] I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas (1.0.5)

`series = pd.Series({'1':1,'2':None,'3':np.nan, '4':''}) `

will create a series which looks like:

    1       1
    2    None
    3     NaN
    4        
    dtype: object

When using the fillna method on this series on index 3 it changes also the entry in index 2!

    series.fillna({'3':0})

This will result in a output as follows:

    1      1
    2    NaN
    3      0
    4       
    dtype: object

**Problem:**
The value at index 2 changed from None to NaN.

**Expected Behaviour:**
The value at index 2 should never be touched when using fillna and specifying the indices you want to alter.


