avoid cryptic error message because of internal function not handling null value

[cderv]

There could be a null value of someone does

format:
  html: null

or

format:
  html:

This is caught by YAML validation, but validation of YAML could happen after format resolution.

fixes #12369

With this PR I correctly get now for quarto preview

❯ quarto preview index.qmd
ERROR: Validation of YAML front matter failed.
ERROR: (line 3, columns 3--11) Field "revealjs" has empty value but it must instead be an object
2: format:
3:   revealjs:
            ~
4: ---

ERROR: Render failed due to invalid YAML.

Stack trace:
    at renderFileInternal (file:///C:/Users/chris/Documents/DEV_R/quarto-cli/src/command/render/render-files.ts:581:19)
    at eventLoopTick (ext:core/01_core.js:175:7)
    at async renderFiles (file:///C:/Users/chris/Documents/DEV_R/quarto-cli/src/command/render/render-files.ts:325:9)
    at async render (file:///C:/Users/chris/Documents/DEV_R/quarto-cli/src/command/render/render-shared.ts:104:18)
    at async renderForPreview (file:///C:/Users/chris/Documents/DEV_R/quarto-cli/src/command/preview/preview.ts:428:24)
    at async render (file:///C:/Users/chris/Documents/DEV_R/quarto-cli/src/command/preview/preview.ts:172:22)
    at async preview (file:///C:/Users/chris/Documents/DEV_R/quarto-cli/src/command/preview/preview.ts:189:18)
    at async Command.actionHandler (file:///C:/Users/chris/Documents/DEV_R/quarto-cli/src/command/preview/cmd.ts:424:7)
    at async Command.execute (https://deno.land/x/cliffy@v1.0.0-rc.3/command/command.ts:1948:7)
    at async Command.parseCommand (https://deno.land/x/cliffy@v1.0.0-rc.3/command/command.ts:1780:14)

Is this the best way to solve #12369 ? IDK but it seems an ok fix to protect against some value that fixupMergeableScalars does not handle

[cscheid]

That is a good, if very strange fix for the error! I will just add a comment pointing here, and then we can close the issue.

[cderv]

Great. I wanted to discuss this and forgot last week.

Very strange fix indeed but definitely solves the problem.

We can still discuss this validation order even after merge.

Thanks for having look into it !