Bug: when there is just one paragraphNode, indentation is not exported on copy

Lexical version: the latest 0.14.5

## Steps To Reproduce

1. make just one paragraphNode with indentation
2. select and paste

Link to code example: can reproduce in the playground

## The current behavior

when there is just one paragraphNode, indentation is not applied on exported html.
It seems because we can't select only one paragraph with indentations.

https://github.com/facebook/lexical/assets/40269597/c8a68fe3-ad19-4ac5-bcab-33f922d54a4d

## The expected behavior

https://github.com/facebook/lexical/assets/40269597/3265c623-453b-41d5-a88e-1bbd51bad365

Reproduce the issue in the Lexical playground with one indented paragraphNode, then inspect the copy/paste path that exports the paragraph to HTML. Compare the current and expected exported markup and verify that indentation is preserved for a single paragraph without changing multi-paragraph behavior.

Written by the indexing model from the issue text.