It's possible to filter out missing data with the Series method dropna.
It returns a Series which consists only of non-null data:
>>> my_city_series
London 8615246.0
Paris 2273305.0
Zurich NaN
Berlin 3562166.0
Stuttgart NaN
Hamburg NaN
dtype: float64
>>> my_city_series.dropna()
London 8615246.0
Paris 2273305.0
Berlin 3562166.0
dtype: float64