test(schema): validate empty schema serialization across all OpenAPI versions

Convert SerializeBasicSchemaAsV3JsonWorks from a single-version test to a theory
that validates empty schema serialization for all supported versions (2.0, 3.0, 3.1, 3.2).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: baywet

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

@@ -204,14 +204,18 @@ public class OpenApiSchemaTests
             }
         };
 
-        [Fact]
-        public async Task SerializeBasicSchemaAsV3JsonWorks()
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi2_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_1)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_2)]
+        public async Task SerializeBasicSchemaAsJsonWorks(OpenApiSpecVersion version)
         {
             // Arrange
             var expected = @"{ }";
 
             // Act
-            var actual = await BasicSchema.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
+            var actual = await BasicSchema.SerializeAsJsonAsync(version);
 
             // Assert
             actual = actual.MakeLineBreaksEnvironmentNeutral();