Handle nullable drag entries in desktop_drop web to avoid zip-drag crashes

[Copilot]

Dragging files from zip archives on web could crash when DataTransferItem.webkitGetAsEntry() returned null.

• Null-safe drop handling

  • Guard against null entries and skip missing items.
  • Fallback to building drop items from getAsFile() when entry metadata is unavailable.
  • Avoid invoking performOperation_web when no items are produced.

• Shared file mapping

  • Added _fileToWebDropItem helper to unify file-to-drop-item conversion for both entry and fallback paths.

Example:

final item = items[index];
final entry = item.webkitGetAsEntry();
if (entry != null) {
  return _entryToWebDropItem(entry);
}
final file = item.getAsFile();
if (file != null) {
  return _fileToWebDropItem(file, relativePath: null);
}
return Future.value(null);

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[desktop_drop_web] Dragging file from zip file crashes</issue_title>
<issue_description>## Reproduce Steps

1. Drag a file from a zip file (windows explorer)
2. Observer error when releasing.

Expected behavior

No errors

Version (please complete the following information):

• Flutter Version: master
• OS: Windows + Chrome
• plugin: [e.g. desktop_drop: 0.7.0]

Stack trace:

errors.dart:274 Uncaught DartError: Unexpected null value.
    at Object.throw_ [as throw] (errors.dart:274:3)
    at Object.nullCheck (operations.dart:950:18)
    at desktop_drop_web.dart:92:46
    at List.generate (core_patch.dart:377:46)
    at desktop_drop_web.dart:90:24
    at Object._callDartFunctionFast1 (js_allow_interop_patch.dart:224:27)
    at ret (js_allow_interop_patch.dart:84:15)
```</issue_description>

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

<comments>
</comments>

• Fixes [desktop_drop_web] Dragging file from zip file crashes #456

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