Context. Audit a dataset — how many NaNs in each column?
Your task. Write nan_counts(df) that returns a Series indexed by column name, with the count of NaN values in each column.
Example. A df with 2 NaN in age and 1 NaN in city → {age: 2, city: 1, name: 0}.
Notes. Include columns with zero NaN. Empty df → Series with one zero per column.