Fix `BottomAppBar` dip without FAB (#104490)

packages/flutter/lib/src/material/bottom_app_bar.dart

@@ -128,8 +128,9 @@ class _BottomAppBarState extends State<BottomAppBar> {
   @override
   Widget build(BuildContext context) {
     final BottomAppBarTheme babTheme = BottomAppBarTheme.of(context);
+    final bool hasFab = Scaffold.of(context).hasFloatingActionButton;
     final NotchedShape? notchedShape = widget.shape ?? babTheme.shape;
-    final CustomClipper<Path> clipper = notchedShape != null
+    final CustomClipper<Path> clipper = notchedShape != null && hasFab
       ? _BottomAppBarClipper(
           geometry: geometryListenable,
           shape: notchedShape,

packages/flutter/test/material/bottom_app_bar_test.dart

@@ -474,6 +474,31 @@ void main() {
       ),
     );
   });
+
+  testWidgets('BottomAppBar does not apply custom clipper without FAB', (WidgetTester tester) async {
+    Widget buildWidget({Widget? fab}) {
+      return MaterialApp(
+        home: Scaffold(
+          floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
+          floatingActionButton: fab,
+          bottomNavigationBar: BottomAppBar(
+            color: Colors.green,
+            shape: const CircularNotchedRectangle(),
+            child: Container(height: 50),
+          ),
+        ),
+      );
+    }
+    await tester.pumpWidget(buildWidget(fab: FloatingActionButton(onPressed: () { })));
+
+    PhysicalShape physicalShape = tester.widget(find.byType(PhysicalShape).at(0));
+    expect(physicalShape.clipper.toString(), '_BottomAppBarClipper');
+
+    await tester.pumpWidget(buildWidget());
+
+    physicalShape = tester.widget(find.byType(PhysicalShape).at(0));
+    expect(physicalShape.clipper.toString(), 'ShapeBorderClipper');
+  });
 }
 
 // The bottom app bar clip path computation is only available at paint time.