How to add `ConstColumn` for `None` value in the Slick

[Unknown Unknown]

There is some table:

case class Thing(name: String, color: Option[String], height: Option[String])

class ThingSchema(t: Tag) extends Table[Thing](t, "things") {
  def name = column[String]("name")
  def color = column[Option[String]]("color")
  def height = column[Option[String]]("height")
  def * = (name, color, height) <> (Thing.tupled, Thing.unapply)
}
val things = TableQuery[ThingSchema]

For example, there are the following data in the things table:

|  name   |   color   | height |
+---------+-----------+--------+
|   n1    |  green    | <null> |
|   n1    |  green    | <null> |
|   n1    |  <null>   | normal |
|   n1    |  <null>   | normal |
|   n1    |  red      | <null> |
|   n2    |  red      | <null> |

I need to get the following result from the above data:

|  name   |   color   | height | size |
+---------+-----------+--------+------+
|   n1    |  green    | <null> |  2   |
|   n1    |  <null>   | normal |  2   |
|   n1    |  red      | <null> |  1   |
|   n2    |  red      | <null> |  1   |

To solve this task I use the following grouping queries:

SELECT name, color, null, count(*) AS size
FROM things
GROUP BY name, color

UNION ALL

SELECT name, null, height, count(*) AS size
FROM things
GROUP BY name, height

I've tried to create this query with the Slick:

val query1 = 
      things.groupBy(t => (t.name, t.color))
            .map { case (name, color) => (name,color,None, g._2.size)} //Error#1

val query2 = 
      things.groupBy(t => (t.name, t.height)) 
            .map { case (name, height) => (name,None,height,g._2.size)} //Error#1

val query = query1 ++ query2

But the above code isn't compiled, because the Slick can't define type for ConstColumn for the None values (see //Error#1 comment in the above code).

This would work for the NOT-null values (such as numbers, strings), but doesn't work for Nullable values which are represented as Option[String]=None for example.

How to use ConstColumn for None values for this case?

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