[List] Change ListItemIcon children type from element to Node (#20577)

* [ListItemIcon] Change `children` PropType to node

* conditional children

Co-authored-by: Olivier Tassinari <olivier.tassinari@gmail.com>

docs/pages/api-docs/list-item-icon.md

@@ -28,7 +28,7 @@ The `MuiListItemIcon` name can be used for providing [default props](/customizat
 
 | Name | Type | Default | Description |
 |:-----|:-----|:--------|:------------|
-| <span class="prop-name required">children&nbsp;*</span> | <span class="prop-type">element</span> |  | The content of the component, normally `Icon`, `SvgIcon`, or a `@material-ui/icons` SVG icon element. |
+| <span class="prop-name">children</span> | <span class="prop-type">node</span> |  | The content of the component, normally `Icon`, `SvgIcon`, or a `@material-ui/icons` SVG icon element. |
 | <span class="prop-name">classes</span> | <span class="prop-type">object</span> |  | Override or extend the styles applied to the component. See [CSS API](#css) below for more details. |
 
 The `ref` is forwarded to the root element.

packages/material-ui/src/ListItemIcon/ListItemIcon.d.ts

@@ -6,7 +6,7 @@ export interface ListItemIconProps
    * The content of the component, normally `Icon`, `SvgIcon`,
    * or a `@material-ui/icons` SVG icon element.
    */
-  children: React.ReactElement;
+  children?: React.ReactNode;
 }
 
 export type ListItemIconClassKey = 'root';

packages/material-ui/src/ListItemIcon/ListItemIcon.js

@@ -49,7 +49,7 @@ ListItemIcon.propTypes = {
    * The content of the component, normally `Icon`, `SvgIcon`,
    * or a `@material-ui/icons` SVG icon element.
    */
-  children: PropTypes.element.isRequired,
+  children: PropTypes.node,
   /**
    * Override or extend the styles applied to the component.
    * See [CSS API](#css) below for more details.