fix mouse-copy inside formatted text

[jcubic]

This fixes the copy (yank) of the text inside formatted Markdown with a mouse. Not sure if the test makes sense here. Because you can't actually test this. You need a real window and a mouse to reproduce.

I use this code to be able to use the mouse to yank text:

(setq x-select-enable-clipboard t)
(setq mouse-drag-copy-region 'non-empty)
(setq x-select-enable-clipboard-manager nil)
;; (setq interprogram-paste-function 'x-cut-buffer-or-selection-value)
(setq interprogram-paste-function 'x-selection-value)
(delete-selection-mode t)

(add-to-list 'yank-excluded-properties 'font)
(add-to-list 'yank-excluded-properties 'font-lock-face)

Before the fix, when you copy the text with the mouse and select it (exactly the formatted text or inside), the text is not copied because the selection is empty.

The keyboard copy works fine. Only the mouse selection (yank) is affected.

Checklist

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• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature.
• I'm making visual changes, so I'm including screenshots so you can view and discuss.
• I've added tests where applicable.
• I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.

[xenodium]

Thank you for the fix