Remove unnecessary nullable types in examples. (flutter#176713)

### Description

- Removes unnecessary `bool?` usage in examples

## Pre-launch Checklist

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there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.


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Author: ksokolovskyi

examples/api/lib/cupertino/switch/cupertino_switch.0.dart

@@ -39,10 +39,10 @@ class _CupertinoSwitchExampleState extends State<CupertinoSwitchExample> {
           // This bool value toggles the switch.
           value: switchValue,
           activeTrackColor: CupertinoColors.activeBlue,
-          onChanged: (bool? value) {
+          onChanged: (bool value) {
             // This is called when the user toggles the switch.
             setState(() {
-              switchValue = value ?? false;
+              switchValue = value;
             });
           },
         ),

examples/api/lib/material/autocomplete/autocomplete.4.dart

@@ -107,7 +107,7 @@ class _AsyncAutocompleteState extends State<_AsyncAutocomplete> {
         ),
         Switch(
           value: _networkEnabled,
-          onChanged: (bool? value) {
+          onChanged: (bool value) {
             setState(() {
               _networkEnabled = !_networkEnabled;
             });

examples/api/lib/material/list_tile/list_tile.3.dart

@@ -62,10 +62,10 @@ class _ListTileExampleState extends State<ListTileExample> {
           title: const Text('Headline'),
           subtitle: Text('Enabled: $_enabled, Selected: $_selected'),
           trailing: Switch(
-            onChanged: (bool? value) {
+            onChanged: (bool value) {
               // This is called when the user toggles the switch.
               setState(() {
-                _enabled = value!;
+                _enabled = value;
               });
             },
             value: _enabled,

examples/api/lib/material/switch_list_tile/switch_list_tile.1.dart

@@ -41,9 +41,9 @@ class _SwitchListTileExampleState extends State<SwitchListTileExample> {
         children: <Widget>[
           SwitchListTile(
             value: switchValue1,
-            onChanged: (bool? value) {
+            onChanged: (bool value) {
               setState(() {
-                switchValue1 = value!;
+                switchValue1 = value;
               });
             },
             title: const Text('Headline'),
@@ -52,9 +52,9 @@ class _SwitchListTileExampleState extends State<SwitchListTileExample> {
           const Divider(height: 0),
           SwitchListTile(
             value: switchValue2,
-            onChanged: (bool? value) {
+            onChanged: (bool value) {
               setState(() {
-                switchValue2 = value!;
+                switchValue2 = value;
               });
             },
             title: const Text('Headline'),
@@ -65,9 +65,9 @@ class _SwitchListTileExampleState extends State<SwitchListTileExample> {
           const Divider(height: 0),
           SwitchListTile(
             value: switchValue3,
-            onChanged: (bool? value) {
+            onChanged: (bool value) {
               setState(() {
-                switchValue3 = value!;
+                switchValue3 = value;
               });
             },
             title: const Text('Headline'),