Bug: Unexpected behaviour when pressing backspace in empty first node

If the selection is in an empty paragraph at the top of a lexical editor AND there are non-empty nodes below, hitting backspace removes the empty node. Text editors usually do nothing instead of deleting the empty paragraph.

Lexical version: 0.17.1

## Steps To Reproduce

1. Nav to the playground https://playground.lexical.dev
2. Place the caret at offset 0
3. Type return
4. Place the caret at offset 0 again, in the empty paragraph
5. Hit backspace

## The current behavior
Backspace in empty paragraph at head of editor deletes the paragraph.

## The expected behavior
Backspace in empty paragraph at head of editor should do nothing.

## Impact of fix
More standard behaviour, aligned with other text editors.

Reproduce the issue in the Lexical playground at https://playground.lexical.dev using the listed steps, then trace the editor's backspace handling for an empty paragraph at the top. Done means backspace leaves that empty paragraph unchanged when non-empty nodes follow it, while preserving normal backspace behavior elsewhere.

Written by the indexing model from the issue text.