Allow saving no plan on namespaces

app/models/ee/namespace.rb

@@ -24,7 +24,7 @@ module EE
       delegate :shared_runners_minutes, :shared_runners_seconds, :shared_runners_seconds_last_reset,
         to: :namespace_statistics, allow_nil: true
 
-      validates :plan, inclusion: { in: EE_PLANS.keys }, allow_nil: true
+      validates :plan, inclusion: { in: EE_PLANS.keys }, allow_blank: true
     end
 
     # Checks features (i.e. https://about.gitlab.com/products/) availabily
@@ -58,7 +58,7 @@ module EE
     def plans
       @ancestors_plans ||=
         if parent_id
-          ancestors.where.not(plan: nil).reorder(nil).pluck('DISTINCT plan') + [plan]
+          ancestors.where.not(plan: [nil, '']).reorder(nil).pluck('DISTINCT plan') + [plan]
         else
           [plan]
         end

app/views/admin/_namespace_plan.html.haml

 .form-group
   = f.label :plan, class: 'control-label'
   .col-sm-10
-    = f.select :plan, options_for_select(Namespace::EE_PLANS.keys.map { |plan| [plan.titleize, plan] }, f.object.plan), {}, class: 'form-control'
+    = f.select :plan, options_for_select(Namespace::EE_PLANS.keys.map { |plan| [plan.titleize, plan] }, f.object.plan),
+      { include_blank: 'No associated plan' },
+      class: 'form-control'

spec/models/ee/namespace_spec.rb

@@ -8,7 +8,7 @@ describe Namespace, models: true do
   it { is_expected.to delegate_method(:shared_runners_minutes).to(:namespace_statistics) }
   it { is_expected.to delegate_method(:shared_runners_seconds).to(:namespace_statistics) }
   it { is_expected.to delegate_method(:shared_runners_seconds_last_reset).to(:namespace_statistics) }
-  it { is_expected.to validate_inclusion_of(:plan).in_array(Namespace::EE_PLANS.keys).allow_nil }
+  it { is_expected.to validate_inclusion_of(:plan).in_array(Namespace::EE_PLANS.keys).allow_blank }
 
   describe '#feature_available?' do
     let(:group) { create(:group, plan: plan_license) }