Fix: initialize undomanager for empty document

[benjaminfrueh]

The first paste action in an empty document was not being tracked by the Yjs UndoManager, making it impossible to undo and not tracking it as a change.

This adds initUndoManagerForEmptyDocument() which primes the undo manager by inserting and deleting a space, then clearing the undo stack. This ensures user actions are properly tracked.

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:white_check_mark: All modified and coverable lines are covered by tests.

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[benjaminfrueh]

After more detailed debugging of the undoManager, I found the root cause and also was able to find a solution. Closing this PR and creating a new one with a fix.

[benjaminfrueh]

Fixed in https://github.com/nextcloud/text/pull/8063