Add presence validation to AttribValue position

The position column is required in the database, but the model was
missing the validation. This commit adds the validation to the
model so the constraint is in sync.

Author: danidoni

src/api/.database_consistency.todo.yml

@@ -66,9 +66,6 @@ AttribValue:
   id:
     PrimaryKeyTypeChecker:
       enabled: false
-  position:
-    NullConstraintChecker:
-      enabled: false
   value:
     LengthConstraintChecker:
       enabled: false

src/api/app/models/attrib_value.rb

@@ -2,6 +2,8 @@ class AttribValue < ApplicationRecord
   acts_as_list scope: :attrib
   belongs_to :attrib, optional: true
 
+  validates :position, presence: true
+
   after_initialize :set_default_value
   before_validation :universal_newlines