Placeholder not shown when another editor's cursor is on the same line

Describe the bug

When the cursor is on an empty line, a placeholder should be displayed. However, in collaborative editing scenarios, the placeholder is not shown if another editor's cursor is present on the same line.

This issue can be observed in the following video:

https://github.com/user-attachments/assets/9c097cac-5c37-4433-a6e3-09e54410fb35

This issue is particularly problematic when editing a completely empty document, as it occurs on the very first line, leading to a confusing user experience.

To Reproduce

1. Open a document in collaborative editing mode with multiple users - https://www.blocknotejs.org/
2. One user places their cursor on an empty line
3. Another user places their cursor on the same empty line
4. Observe that the placeholder text is not displayed

Expected behavior:
The placeholder should be visible on empty lines regardless of other editors' cursor positions.

Actual behavior:
The placeholder is hidden when multiple cursors are present on the same empty line.

Misc

• Node version:
• Package manager:
• Browser:
• I'm a sponsor and would appreciate if you could look into this sooner than later 💖

Reproduce the issue at the BlockNote collaborative editing demo with two users placing cursors on the same empty line. Trace the placeholder rendering and collaborative cursor handling to determine why the placeholder disappears, then verify that it remains visible on empty lines with other cursors present.