fix: Validate schema property is not null

Author: cliedeman

src/Microsoft.OpenApi/Properties/SRResource.Designer.cs

@@ -219,6 +219,9 @@
   <data name="Validation_SchemaRequiredFieldListMustContainThePropertySpecifiedInTheDiscriminator" xml:space="preserve">
     <value>Schema {0} must contain property specified in the discriminator {1} in the required field list.</value>
   </data>
+  <data name="Validation_SchemaPropertyObjectRequired" xml:space="preserve">
+    <value>Schema {0} property {1} is null.</value>
+  </data>
   <data name="Validation_StringMustBeEmailAddress" xml:space="preserve">
     <value>The string '{0}' MUST be in the format of an email address.</value>
   </data>

src/Microsoft.OpenApi/Properties/SRResource.resx

@@ -11,6 +11,31 @@ namespace Microsoft.OpenApi
     [OpenApiRule]
     public static class OpenApiSchemaRules
     {
+        /// <summary>
+        /// Validates Schema Property has value
+        /// </summary>
+        public static ValidationRule<IOpenApiSchema> ValidateSchemaPropertyHasValue =>
+            new(nameof(ValidateSchemaPropertyHasValue),
+                (context, schema) =>
+                {
+                    if (schema.Properties is not null)
+                    {
+                        foreach (var property in schema.Properties)
+                        {
+                            if (property.Value is null)
+                            {
+                                context.Enter(property.Key);
+                                context.CreateError(nameof(ValidateSchemaPropertyHasValue),
+                                    string.Format(SRResource.Validation_SchemaPropertyObjectRequired,
+                                        schema is OpenApiSchemaReference { Reference: not null } schemaReference
+                                            ? schemaReference.Reference.Id
+                                            : string.Empty, property.Key));
+                                context.Exit();
+                            }
+                        }
+                    }
+                });
+        
         /// <summary>
         /// Validates Schema Discriminator
         /// </summary>

src/Microsoft.OpenApi/Validations/Rules/OpenApiSchemaRules.cs

@@ -174,6 +174,44 @@ public void ValidateDefaultShouldNotHaveDataTypeMismatchForComplexSchema()
             // Assert
             Assert.NotEmpty(validator.Warnings);
         }
+        
+        [Fact]
+        public void ValidateNullProperty()
+        {
+            // Arrange
+            var components = new OpenApiComponents
+            {
+                Schemas = new Dictionary<string, IOpenApiSchema>
+                {
+                    {
+                        "schema1",
+                        new OpenApiSchema
+                        {
+                            Properties = new Dictionary<string, IOpenApiSchema>
+                            {
+                                ["property1"] = null,
+                            }
+                        }
+                    }
+                }
+            };
+
+            // Act
+            var defaultRuleSet = ValidationRuleSet.GetDefaultRuleSet();
+            defaultRuleSet.Add(typeof(IOpenApiSchema), OpenApiNonDefaultRules.SchemaMismatchedDataType);
+            var validator = new OpenApiValidator(defaultRuleSet);
+            var walker = new OpenApiWalker(validator);
+            walker.Walk(components);
+
+            // Assert
+            Assert.NotEmpty(validator.Errors);
+            Assert.Equivalent(new List<OpenApiValidatorError>
+            {
+                new OpenApiValidatorError(nameof(OpenApiSchemaRules.ValidateSchemaPropertyHasValue),"#/schemas/schema1/property1",
+                    string.Format(SRResource.Validation_SchemaPropertyObjectRequired,
+                        string.Empty, "property1"))
+            }, validator.Errors);
+        }
 
         [Fact]
         public void ValidateSchemaRequiredFieldListMustContainThePropertySpecifiedInTheDiscriminator()