fix: an empty security requirement should not result in an object during serialization

Signed-off-by: Vincent Biret <[email protected]>

Author: baywet

test/Microsoft.OpenApi.Tests/Models/OpenApiSecurityRequirementTests.cs

@@ -71,7 +71,7 @@ public class OpenApiSecurityRequirementTests
         public async Task SerializeBasicSecurityRequirementAsV3JsonWorks()
         {
             // Arrange
-            var expected = @"{ }";
+            var expected = string.Empty;
 
             // Act
             var actual = await BasicSecurityRequirement.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);