It's possible to filter out missing data with the Series method dropna. It returns a Series which consists only of non-null data:
print(my_city_series.dropna())
If we call fillna with a dict, we can provide the appropriate data:
London 8615246.0
Paris 2273305.0
Berlin 3562166.0
Hamburg 1760433.0
dtype: float64
missing_cities = {"Stuttgart":597939, "Zurich":378884}
my_city_series.fillna(missing_cities)
London 8615246.0
Paris 2273305.0
Zurich 378884.0
Berlin 3562166.0
Stuttgart 597939.0
Hamburg 1760433.0
dtype: float64