action_sheet: Implement BottomSheetEmptyContentPlaceholder, to use soon

Author: chrisbobbe

lib/widgets/action_sheet.dart

@@ -181,6 +181,39 @@ class BottomSheetHeader extends StatelessWidget {
   }
 }
 
+/// A "no content here" message for when a bottom sheet has no content to show.
+///
+/// Show this below a [BottomSheetHeader] if present.
+///
+/// See also:
+///  * [PageBodyEmptyContentPlaceholder], for a similar element to use in
+///    pages on the home screen.
+// TODO(design) we don't yet have a design for this;
+//   it was ad-hoc and modeled on [PageBodyEmptyContentPlaceholder].
+class BottomSheetEmptyContentPlaceholder extends StatelessWidget {
+  const BottomSheetEmptyContentPlaceholder({super.key, required this.message});
+
+  final String message;
+
+  @override
+  Widget build(BuildContext context) {
+    final designVariables = DesignVariables.of(context);
+
+    return Padding(
+        padding: EdgeInsets.fromLTRB(24, 48, 24, 16),
+        child: Align(
+          alignment: Alignment.topCenter,
+          child: Text(
+            textAlign: TextAlign.center,
+            style: TextStyle(
+              color: designVariables.labelSearchPrompt,
+              fontSize: 17,
+              height: 23 / 17,
+            ).merge(weightVariableTextStyle(context, wght: 500)),
+            message)));
+  }
+}
+
 /// A button in an action sheet.
 ///
 /// When built from server data, the action sheet ignores changes in that data;