Returns null_replacement if the given expression is null; otherwise the expression is
This function returns a data type that is very specific in terms of nullability. The
returned type is the common type of both arguments but only nullable if the
null_replacement is nullable.
The function allows to pass nullable columns into a function or table that is declared
with a NOT NULL constraint.
e.g. col(“nullable_column”).if_null(5) returns never null.