Remove check on HasMultipleTypes as WriteUnifiedType will do this check as well

Author: desjoerd

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -884,15 +884,7 @@ private static (IList<IOpenApiSchema>? effective, JsonSchemaType? inferredType,
                     commonType |= schema.Type.GetValueOrDefault() & ~JsonSchemaType.Null;
                 }
 
-                if (HasMultipleTypes(commonType) || commonType == 0)
-                {
-                    return (nonNullSchemas, null, true);
-                }
-                else
-                {
-                    // Single common type
-                    return (nonNullSchemas, commonType, true);
-                }
+                return (nonNullSchemas, commonType, true);
             }
             else
             {

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

@@ -844,7 +844,7 @@ public async Task SerializeOneOfWithNullAndMultipleSchemasAsV3ShouldMarkItAsNull
                 {
                     new OpenApiSchema { Type = JsonSchemaType.Null },
                     new OpenApiSchema { Type = JsonSchemaType.String },
-                    new OpenApiSchema { Type = JsonSchemaType.Number }
+                    new OpenApiSchema { Type = JsonSchemaType.Number },
                 }
             };