Do not allow issue_id to be nil

In requirement model

ee/app/models/requirements_management/requirement.rb

@@ -27,7 +27,7 @@ module RequirementsManagement
     # This will be removed in https://gitlab.com/gitlab-org/gitlab/-/issues/329432
     belongs_to :requirement_issue, class_name: 'Issue', foreign_key: :issue_id, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 
-    validates :issue_id, uniqueness: true, allow_nil: true
+    validates :issue_id, uniqueness: true
 
     has_many :test_reports, inverse_of: :requirement
     has_many :recent_test_reports, -> { order(created_at: :desc) }, class_name: 'TestReport', inverse_of: :requirement

ee/spec/models/requirements_management/requirement_spec.rb

@@ -46,13 +46,7 @@ RSpec.describe RequirementsManagement::Requirement do
   describe 'validations' do
     subject { build(:requirement) }
 
-    context 'with requirement issue' do
-      let(:ri) { create(:requirement_issue) }
-
-      subject { build(:requirement, requirement_issue: ri) }
-
-      it { is_expected.to validate_uniqueness_of(:issue_id).allow_nil }
-    end
+    it { is_expected.to validate_uniqueness_of(:issue_id) }
 
     it 'is limited to a unique requirement_issue' do
       requirement_issue = create(:requirement_issue)