empty image_files left behind with output-dir

Bug description

Using Quarto 1.3.340 and macOS.

Given the Quarto configuration that declares an output directory:

project:
  output-dir: out

and a single index.qmd file:

---
title: nothing important
---

this is some content.

Rendering this content produces the files:

./index_files/ (an empty directory)
./out/index.html
./out/index_files/

There is an index_files/ directory left behind alongside the out/ output directory.

[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.1: OK
      Dart Sass version 1.55.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.3.340
      Path: /Applications/RStudio.app/Contents/Resources/app/quarto/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.9.6
      Path: /Library/Developer/CommandLineTools/usr/bin/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 3.6.3
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/3.6/Resources/library
      knitr: 1.42.8
      rmarkdown: 2.20

[✓] Checking Knitr engine render......OK

Checklist

• Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
• Please format your issue so it is easier for us to read the bug report.
• Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
• Please document the operating system you're running. If on Linux, please provide the specific distribution.
• Please provide the output of quarto check so we know which version of quarto and its dependencies you're running.

Reproduce the report with the shown project output-dir configuration and index.qmd, then inspect the Quarto render path that creates index_files/. Confirm that rendering writes only the intended output under out/ and does not leave an empty index_files/ directory beside it.