Fix crash on empty paths in onAssetsChanged

[Copilot]

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

Original prompt

---

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 [BUG] Bad state: No element #736

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[AlexV525]

@copilot write changelog

[Copilot]

@copilot write changelog

Added changelog entry in commit abacd02.