You can define your own function to avoid NaN
In [36]: def _sum(x):
....: if len(x) == 0: return 0
....: else: return sum(x)
....:
In [37]: s.resample('M', how=_sum)
Out[37]:
2013-01-31 3
2013-02-28 0
2013-03-31 3
Freq: M, dtype: int64