NULL is a data type and is not a scalar
While NULL is listed as a data type the consequences of this is not triivial. Most importantly, this is the reason why it is not considered a scalar.
is_scalar($variable) is a shorthand for
is_bool($variable) || is_string($variable) || is_int($variable) || is_float($variable) and NULL is none of those. It "feels" like NULL should be a scalar -- Wikipedia says "In computing, a scalar is any non-composite value". This is just one of these decisions PHP made (and I am not sure this was a conscious decision by anyone, it might have been a simple oversight of not adding is_null to the above) and you need to memorize: NULL is not a scalar.