Throw a helpful error when returning an empty list in pageListBuilder

[TahaTesser]

Description

When you return a list empty in the pageListBuilder (this could happen if the provided list is empty or not initialized), it throws an unhelpful error.

flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following RangeError was thrown building ValueListenableBuilder<int>(dirty, state:
flutter: _ValueListenableBuilderState<int>#93302):
flutter: RangeError (index): Invalid value: Valid value range is empty: 0

Instead, throw an error that says pageListBuilder is empty and at least one page should be provided.

Code sample

expand to view the code sample

import 'package:flutter/material.dart';
import 'package:wolt_modal_sheet/wolt_modal_sheet.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
          body: Center(
            child: Builder(
              builder: (context) {
                return ElevatedButton(
                  onPressed: () {
                    WoltModalSheet.show(
                      context: context,
                      pageListBuilder: (context) {
                        return <WoltModalSheetPage>[

                        ];
                      },
                    );
                  },
                  child: const Text('Open sheet'),
                );
              }
            ),
          )
        ),
      );
  }
}

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