Require the variable key to be set and unique

CHANGELOG

@@ -12,6 +12,7 @@ v7.14.0 (unreleased)
   - Added support for CI skipped status
   - Fix broken yaml error saving
   - Rename type(s) to stage(s)
+  - Require variable keys to be not-empty and unique
   
 v7.13.1
   - Fix: user could steal specific runner

app/models/variable.rb

@@ -14,5 +14,8 @@
 class Variable < ActiveRecord::Base
   belongs_to :project
+  validates_presence_of :key
+  validates_uniqueness_of :key
+
   attr_encrypted :value, mode: :per_attribute_iv_and_salt, key: GitlabCi::Application.secrets.db_key_base
 end