def empty_str_to_null(s):
    """Convert empty strings to None (null)"""
    s.loc[s.str.strip().str.len() == 0] = None
    return s

foo = pd.Series(np.repeat([1,2,3,'',None,np.NaN, '  '],1E6))

>>> %time bar = empty_str_to_null(foo)

CPU times: user 7.67 s, sys: 260 ms, total: 7.93 s
Wall time: 8.38 s

def empty_str_to_null_slicer(s):
    a = s.values.astype(str)
    # slicer_vectorized from https://stackoverflow.com/a/39045337/
    mask = (slicer_vectorized(a,0,1)==' ') | (a=='')
    s[mask] = None
    return s

In [245]: s = pd.Series(np.repeat([1,'',' ',None,np.NaN],2))

In [246]: s
Out[246]:
0       1
1       1
2
3
4
5
6    None
7    None
8     NaN
9     NaN
dtype: object

In [247]: a = s.values.astype(str)
     ...: mask = (slicer_vectorized(a,0,1)==' ') | (a=='')
     ...: s[mask] = None
     ...:

In [248]: s
Out[248]:
0       1
1       1
2    None
3    None
4    None
5    None
6    None
7    None
8     NaN
9     NaN
dtype: object

# Original approach
def empty_str_to_null(s0):
    s = s0.copy()
    """Convert empty strings to None (null)"""
    s.loc[s.str.strip().str.len() == 0] = None
    return s

# Proposed approach
def empty_str_to_null_slicer(s0):
    s = s0.copy()
    a = s.values.astype(str)
    # slicer_vectorized from https://stackoverflow.com/a/39045337/3293881
    mask = (slicer_vectorized(a,0,1)==' ') | (a=='')
    s[mask] = None
    return s

In [228]: foo = pd.Series(np.repeat([1,'',' ',None,np.NaN],1E6))

In [229]: %timeit empty_str_to_null(foo)
1 loop, best of 3: 4.17 s per loop

In [230]: %timeit empty_str_to_null_slicer(foo)
1 loop, best of 3: 573 ms per loop