Fix warning in namespace model

This silences this warning:
    warning: instance variable @plan_name not initialized

Also test explicitly that blank plan names are valid.

ee/app/models/ee/namespace.rb

@@ -329,7 +329,7 @@ module EE
     private
 
     def validate_plan_name
-      if @plan_name.present? && PLANS.exclude?(@plan_name) # rubocop:disable Gitlab/ModuleWithInstanceVariables
+      if defined?(@plan_name) && @plan_name.present? && PLANS.exclude?(@plan_name) # rubocop:disable Gitlab/ModuleWithInstanceVariables
         errors.add(:plan, 'is not included in the list')
       end
     end

ee/spec/models/namespace_spec.rb

@@ -117,12 +117,18 @@ describe Namespace do
       end
 
       context 'with an invalid plan name' do
-        it 'is invalid' do
+        it 'is invalid when `unknown`' do
           group.plan = 'unknown'
 
           expect(group).not_to be_valid
           expect(group.errors[:plan]).to include('is not included in the list')
         end
+
+        it 'is valid for blank strings' do
+          group.plan = ' '
+
+          expect(group).to be_valid
+        end
       end
     end