Pandas: How can I update values in dataframe?

Question

I have two spreadsheets where one is updating the other.

How can I update this data using pandas?

Example, where 'b' updates 'a':

a = {'campo': ['a', 'b', 'c'], 'valor': ["", None, 1]}
b = {'campo': ['a', 'b', 'd'], 'valor': [1, 2, 1]}

Dataframes:

dfa = pd.DataFrame(a)

  campo valor
0     a      
1     b  None
2     c     1


dfb = pd.DataFrame(b)

  campo  valor
0     a      1
1     b      2
2     d      1

Expected result:

c = {'campo': ['a', 'b', 'c', 'd'], 'valor': [1, 2, 1, 1]}

  campo  valor
0     a      1
1     b      2
2     c      1
3     d      1

I tried several pandas.Concat and pandas.merge.. available in https://pastebin.com/embed_iframe/X4WjuLHU