Update if parameter is not null or empty

Using just IsNull your query would look something like…

UPDATE [Users]
SET FirstName = IsNull(@FirstName, FirstName),
City = IsNull(@City, City)

DECLARE @value AS INT = Null
SELECT CASE WHEN ISNULL(@value, 2)=0 THEN Null Else ISNULL(@value, 2) END

will Update the row with the param value if they are NOT null, otherwise update it to itself aka change nothing.