Fix crash on empty paths in onAssetsChanged

[Copilot]

• [x] Analyze the bug report and understand the issue
• [x] Locate the problematic code in asset_picker_builder_delegate.dart line 1084
• [x] Identify the root cause: accessing .first on potentially empty paths list
• [x] Fix the issue by adding safety check for empty paths
• [x] Run existing tests to ensure no regression (all 13 tests passed)
• [x] Run flutter analyze (no issues found)
• [x] Run code review (no issues found)
• [x] Run CodeQL security check (no issues found)
• [x] Add changelog entry

Original prompt

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This section details on the original issue you should resolve

<issue_title>[BUG] Bad state: No element</issue_title> <issue_description>Describe the bug 我们在Firebase上看到了使用了 wechat_assets_picker: 9.7.0 版本产生的一些Bug。而之前的 wechat_assets_picker: 9.0.0 并没有。

How to reproduce 暂无复现步骤

Expected behavior 期望不要发生此类异常

Screenshots (If contains)

Version information

• Device: 如图
• OS: Android 15+
• Package Version: v9.7.0
• Flutter Version: 3.29.3

Additional context

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes fluttercandies/flutter_wechat_assets_picker#736

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[AlexV525]

@copilot write changelog

[Copilot]

@copilot write changelog

Added changelog entry in commit abacd02.