email: lack of subject causes stack trace

[aronatkins]

Bug description

The email format errs when the document does not contain a .subject block.

Steps to reproduce

---
title: customized
format: email
---

Not the message.

::: {.email}

The message.

:::

Error running filter /Applications/quarto/share/filters/main.lua:
/Applications/quarto/share/filters/main.lua:7233: attempt to concatenate a nil value (upvalue 'subject')
stack traceback:
	[C]: in ?
	[C]: in method 'walk'
	/Applications/quarto/share/filters/main.lua:224: in function </Applications/quarto/share/filters/main.lua:214>
	(...tail calls...)
	/Applications/quarto/share/filters/main.lua:936: in local 'callback'
	/Applications/quarto/share/filters/main.lua:954: in upvalue 'run_emulated_filter_chain'
	/Applications/quarto/share/filters/main.lua:990: in function </Applications/quarto/share/filters/main.lua:987>
stack traceback:
	/Applications/quarto/share/filters/main.lua:224: in function </Applications/quarto/share/filters/main.lua:214>
	(...tail calls...)
	/Applications/quarto/share/filters/main.lua:936: in local 'callback'
	/Applications/quarto/share/filters/main.lua:954: in upvalue 'run_emulated_filter_chain'
	/Applications/quarto/share/filters/main.lua:990: in function </Applications/quarto/share/filters/main.lua:987>

If the document contains a .subject block, it does not err.

---
title: customized
format: email
---

Not the message.

::: {.email}

::: {.subject}
The subject.
:::

The message.

:::

Expected behavior

When the document does not include a .subject block, the render should still succeed.

The .output_metadata.json file should include a value for the rsc_email_subject field only when the document is providing a subject. When there is no custom subject, the JSON file should either have an empty string value or omit the field.

When Connect renders a document, it provides the RSC_EMAIL_SUBJECT environment variable. This contains the default subject in case folks want to use that value when computing an overridden value. Quarto does not need to interpret RSC_EMAIL_SUBJECT or provide a default subject.

Actual behavior

Stack trace

Your environment

No response

Quarto check output

Quarto 1.4.549
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.11: OK
      Dart Sass version 1.69.5: OK
      Deno version 1.37.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.4.549
      Path: /Applications/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/aron/Library/TinyTeX/bin/universal-darwin
      Version: 2022

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

[✓] Checking Python 3 installation....OK
      Version: 3.11.7
      Path: /usr/local/opt/python@3.11/bin/python3.11
      Jupyter: (None)

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

[✓] Checking R installation...........OK
      Version: 4.3.1
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/library
      knitr: 1.45
      rmarkdown: 2.25

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

[cderv]

Just a note from @aronatkins : This would be could to add to a 1.4 patch release.

I added the backport label to indicate this

[cscheid]

@rich-iannone @cderv Can either of you backport this to v1.4?

[cderv]

I'll do it. @rich-iannone I can show you how this works.

[cderv]

@cscheid this is now in v1.4 branch. I updated changelog there too.