Make nilable methods only available if column is nilable

`is_nil`, `is_not_nil` and `nilable_eq` should only be available if column is nilable. If not we should still define the methods but raise at compile time saying the column is not nilable

Start by locating the definitions of is_nil, is_not_nil, and nilable_eq, then trace how column nilability is represented and checked at compile time. The work is done when these methods remain defined for non-nilable columns but produce the specified compile-time error, while nilable columns retain the methods without that error.

Written by the indexing model from the issue text.