Fix codeQL warnings

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiComponents.cs

@@ -339,9 +339,6 @@ private void RenderComponents(IOpenApiWriter writer, Action<IOpenApiWriter, IOpe
             writer.WriteStartObject();
             if (loops.TryGetValue(typeof(OpenApiSchema), out List<object> schemas))
             {
-                var openApiSchemas = schemas.Cast<OpenApiSchema>().Distinct().ToList()
-                       .ToDictionary(k => k.Reference.Id);
-
                 writer.WriteOptionalMap(OpenApiConstants.Schemas, Schemas, callback);
             }
             writer.WriteEndObject();

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -667,12 +667,9 @@ public override void Visit(IOpenApiReferenceable referenceable)
         public override void Visit(OpenApiSchema schema)
         {
             // This is needed to handle schemas used in Responses in components
-            if (schema.Reference != null)
+            if (schema.Reference != null && !Schemas.ContainsKey(schema.Reference.Id))
             {
-                if (!Schemas.ContainsKey(schema.Reference.Id))
-                {
-                    Schemas.Add(schema.Reference.Id, schema);
-                }
+                Schemas.Add(schema.Reference.Id, schema);
             }
             base.Visit(schema);
         }

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

@@ -61,12 +61,9 @@ public static void ValidateDataTypeMismatch(
 
             // Before checking the type, check first if the schema allows null.
             // If so and the data given is also null, this is allowed for any type.
-            if (nullable)
+            if (nullable && jsonElement.ValueKind is JsonValueKind.Null)
             {
-                if (jsonElement.ValueKind is JsonValueKind.Null)
-                {
-                    return;
-                }
+                return;
             }
 
             if (type == "object")