'How to add a default value for a column if it is null OR blank? i.e. a whitespace like ''

I added a constraint for one of my columns, the idea being, if it is blank or null, the default value will be unknown.

ALTER TABLE Persons
ADD CONSTRAINT df_City
DEFAULT 'Unknown' FOR City;

I uploaded some samples, and the constraint works with null, but not with ''.

How can I make it work with ''?

SQL Server 14.0.1000.169

sql-server

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