website: Optional object attributes handling of null

Previously we didn't describe the interaction between default values and
callers explicitly passing "null".

We treat an explicit null as the same as omitting the attribute when
applying defaults, because that then allows callers to use the typical
pattern for conditional assignment, using explicit null as a fallback
to the module's defined default without having to duplicate that default:
    example = var.foo ? "hello" : null

website/docs/language/expressions/type-constraints.mdx

@@ -260,7 +260,7 @@ value and thus perform no type conversion whatsoever.
 
 ## Optional Object Type Attributes
 
--> **Note:** Optional type attributes are supported in Terraform v1.3 and later.
+-> **Note:** Optional object type attributes are supported only in Terraform v1.3 and later.
 
 Terraform typically returns an error when it does not receive a value for specified object attributes. When you mark an attribute as optional, Terraform instead inserts a default value for the missing attribute. This allows the receiving module to describe an appropriate fallback behavior.
 
@@ -281,6 +281,8 @@ The `optional` modifier takes one or two arguments.
 specifies the type of the attribute.
 - **Default:** (Optional) The second argument defines the default value that Terraform should use if the attribute is not present. This must be compatible with the attribute type. If not specified, Terraform uses a `null` value of the appropriate type as the default.
 
+An optional attribute with a non-`null` default value is guaranteed to never have the value `null` within the receiving module. Terraform will substitute the default value both when a caller omits the attribute altogether and when a caller explicitly sets it to `null`, thereby avoiding the need for additional checks to handle a possible null value.
+
 Terraform applies object attribute defaults top-down in nested variable types. This means that Terraform applies the default value you specify in the `optional` modifier first and then later applies any nested default values to that attribute.
 
 ### Example: Nested Structures with Optional Attributes and Defaults
@@ -383,3 +385,33 @@ tolist([
   },
 ])
 ```
+
+### Example: Conditionally setting an optional attribute
+
+Sometimes the decision about whether or not to set a value for an optional argument needs to be made dynamically based on some other data. In that case, the calling `module` block can use a conditional expression with `null` as one of its result arms to represent dynamically leaving the argument unset.
+
+With the `variable "buckets"` declaration shown in the previous section, the following example conditionally overrides the `index_document` and `error_document` settings in the `website` object based on a new variable `var.legacy_filenames`:
+
+```hcl
+variable "legacy_filenames" {
+  type     = bool
+  default  = false
+  nullable = false
+}
+
+module "buckets" {
+  source = "./modules/buckets"
+
+  buckets = [
+    {
+      name = "maybe_legacy"
+      website = {
+        error_document = var.legacy_filenames ? "ERROR.HTM" : null
+        index_document = var.legacy_filenames ? "INDEX.HTM" : null
+      }
+    },
+  ]
+}
+```
+
+When `var.legacy_filenames` is set to `true`, the call will override the document filenames. When it is `false`, the call will leave the two filenames unspecified, thereby allowing the module to use its specified default values.