chore: clean up code and add test

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/OpenApiYamlReader.cs

@@ -126,8 +126,7 @@ static JsonNode LoadJsonNodesFromYamlDocument(TextReader input)
             yamlStream.Load(input);
             if (yamlStream.Documents.Any())
             {
-                var yamlDocument = yamlStream.Documents[0];
-                return yamlDocument.ToJsonNode();
+                return yamlStream.Documents[0].ToJsonNode();
             }
 
             throw new InvalidOperationException("No documents found in the YAML stream.");

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiDocumentTests.cs

@@ -11,8 +11,8 @@
 using Microsoft.OpenApi.Writers;
 using Xunit;
 using VerifyXunit;
-using VerifyTests;
 using Microsoft.OpenApi.Models.Interfaces;
+using System;
 
 namespace Microsoft.OpenApi.Readers.Tests.V31Tests
 {
@@ -539,5 +539,11 @@ public async Task ParseDocumentWith31PropertiesWorks()
             // Assert
             await Verifier.Verify(actual);
         }
+
+        [Fact]
+        public void ParseEmptyMemoryStreamThrowsAnArgumentException()
+        {
+            Assert.Throws<ArgumentException>(() => OpenApiDocument.Load(new MemoryStream()));
+        }
     }
 }