docs: add documentation about emptyValue (#4174)

Closes #4070

packages/dnb-design-system-portal/src/docs/uilib/extensions/forms/about-fields.mdx

@@ -31,13 +31,17 @@ Here is a list of [all available fields](/uilib/extensions/forms/all-fields/).
 
 **Table of Contents**
 
-- [Base fields](#base-fields)
-- [Feature fields](#feature-fields)
-- [More info](#more-info)
-
-  - [Standardized properties](#standardized-properties)
-  - [Controlled \& Uncontrolled](#controlled--uncontrolled)
-  - [Creating custom field components](#creating-custom-field-components)
+- [About fields](#about-fields)
+  - [Base fields](#base-fields)
+  - [Feature fields](#feature-fields)
+  - [More info](#more-info)
+    - [Empty value](#empty-value)
+    - [Standardized properties](#standardized-properties)
+    - [Controlled \& Uncontrolled](#controlled--uncontrolled)
+    - [Creating custom field components](#creating-custom-field-components)
+    - [Basic field usage](#basic-field-usage)
+    - [Complete state management and local custom fields](#complete-state-management-and-local-custom-fields)
+  - [First steps](#first-steps)
 
 ## Base fields
 
@@ -58,6 +62,14 @@ On top of these, a number of [feature fields](#feature-fields) have been built t
 
 ## More info
 
+### Empty value
+
+Every field has an `emptyValue` property that can be used to define what value should be used when the user does not enter any value.
+
+When a user removes its content from e.g. a string input field, the value will be set to the `emptyValue`.
+
+- The default `emptyValue` value is `undefined`.
+
 ### Standardized properties
 
 All input component has a fixed set of properties that make it possible to build more complex standardized functionality around them. The most important ones here are `value` and `onChange` (can be async). Value expects values in the given data type, so for example `Field.Number` expects a `value` of the type `number`, and will give a type error in Typescript if it e.g. receives a number in a `string`. The callback function submitted to `onChange` will always receive the value of the corresponding type as the first argument.