Multi document YAML doesn't work with leading and trailing delimiters

Description

In #388836 (comment 1316000941) we got user report about their CI configuration not working on GitLab. We disabled ci_multi_doc_yaml feature flag yesterday to resolve the customer issue.

It appears that our new mechanism does not support following config:

---                                                                                                              
test:                                                                                                            
  script: echo "ok"                                                                                              
---  

This should be a valid syntax, but the following spec is failing:

diff --git a/spec/lib/gitlab/config/loader/multi_doc_yaml_spec.rb b/spec/lib/gitlab/config/loader/multi_doc_yaml_spec.rb
index f63aacecce6a..71252cd3c852 100644
--- a/spec/lib/gitlab/config/loader/multi_doc_yaml_spec.rb
+++ b/spec/lib/gitlab/config/loader/multi_doc_yaml_spec.rb
@@ -6,6 +6,26 @@
   let(:loader) { described_class.new(yml, max_documents: 2) }
 
   describe '#load!' do
+    context 'when a single document with leading and trailing delimiter is used' do
+      let(:yml) do
+        <<~YAML
+        ---
+        test:
+          script: echo "ok"
+        ---
+        YAML
+      end
+
+      it 'returns a single document' do
+        docs = loader.load!
+
+        expect(docs.size).to eq 1
+        expect(docs).to contain_exactly(
+          { test: { script: 'echo "ok"' } }
+        )
+      end
+    end
+
     context 'when a simple single delimiter is being used' do
       let(:yml) do
         <<~YAML

/cc @avielle @fabiopitino @marknuzzo