[desktop_drop] On web, should also handle dragging images from other browser tab

[crizant]

For example, I have two browser windows opened, one is my web app, another is a website full of images.
Then I drag one of the images into my web app, the onDragDone callback returns an empty list.

I have also tested the main branch, it throws the following error:
Error: Expected a value of type 'Entry', but got one of type 'Null'

Expected behavior

The dropped image should be handled without errors.

Version (please complete the following information):

• Flutter Version: 3.16.7
• OS: Chrome browser on MacOS
• plugin: desktop_drop 0.4.4

[crizant]

I have opened a pull request: #317