refactor(react): update tab-bar requirement on tabs (#29868)

Issue number: N/A

---------

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

As mentioned in this
[PR](https://github.com/ionic-team/ionic-docs/pull/3797), React
`IonTabs` requires `IonTabBar` do be a child, else it doesn't render and
throws an error.

Angular, JS, and Vue doesn't have this requirement.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

I didn't see any reason why React does not mimic the other frameworks.
In order to keep consistency, I've updated the React tabs. This would
allow `ion-tabs` and `ion-tab-bar` can be used as standalone elements as
mentioned in the [docs](https://ionicframework.com/docs/api/tabs).

- React follows the same structure as the other frameworks: `IonTabs`
doesn't require `IonTabBar` to be a child to render.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

Dev build: 8.3.1-dev.11726159792.1a6f49de

How to test:
1. Create a Ionic React app through the Ionic CLI with tabs as the
starter
2. Run the app
3. Comment out the `IonTabBar`
4. Notice that the `IonTabs` does not render
5. Notice that there's an error in the console: "IonTabs needs a
IonTabBar"
6. Install the dev build: `npm install
@ionic/react@8.3.1-dev.11726159792.1a6f49de`
7. Make sure the `IonTabBar` is still commented out
8. Verify that `IonTabs` renders
9. Verify that there isn't an error in the console

packages/react/src/components/navigation/IonTabs.tsx

@@ -165,9 +165,6 @@ export const IonTabs = /*@__PURE__*/ (() =>
       if (outlet && hasTab) {
         throw new Error('IonTabs cannot contain an IonRouterOutlet and an IonTab at the same time');
       }
-      if (!tabBar) {
-        throw new Error('IonTabs needs a IonTabBar');
-      }
 
       if (hasTab) {
         return <IonTabsInner {...this.props}></IonTabsInner>;
@@ -223,11 +220,11 @@ export const IonTabs = /*@__PURE__*/ (() =>
             </PageManager>
           ) : (
             <div className={className ? `${className}` : 'ion-tabs'} {...props} style={hostStyles}>
-              {tabBar.props.slot === 'top' ? tabBar : null}
+              {tabBar?.props.slot === 'top' ? tabBar : null}
               <div style={tabsInner} className="tabs-inner">
                 {outlet}
               </div>
-              {tabBar.props.slot === 'bottom' ? tabBar : null}
+              {tabBar?.props.slot === 'bottom' ? tabBar : null}
             </div>
           )}
         </IonTabsContext.Provider>