Null in sql_table constraint

[landmaj]

When null is used in a list of sql_table constraints, it causes an invisible constraint in the rendered diagram.

table: {
  shape: sql_table
  invisible: {constraint: [foreign_key; null; next]}
  ok: {constraint: null}
}

It seems like null is never rendered, which is fine, you can double quote it to get the expected result. However, when used in a list of constraints, it results in an "invisible" constraint.

The documentation is also missing any mention of this behaviour (requiring double quotes around null).