fix: avoid crash when exporting an empty document

[SAY-5]

Summary

Closes #460.

`formatTXT.file` and `formatMD.file` both interpolate
`title.toUpperCase()` directly into their template literals:

${center(title.toUpperCase()) ?? ""}      // formatTXT
# ${title.toUpperCase() ?? ""}             // formatMD

The trailing `?? ""` only fires after the method call, so when
`title` is undefined (a brand-new or otherwise empty mawe document
that has no `<title>` in its head), the call throws
`TypeError: Cannot read properties of undefined (reading 'toUpperCase')`
before the fallback can be reached.

That throw bubbles out of the Preview `flattedFormat` call, which is
why opening the Export view on an empty document crashes — it's the
TXT/MD path the issue was filed against, but every render of the
preview stack hits the same code in those two formats.

Fix

Render the title line the same way `subtitle` is already handled:
emit it only when the value is present, and skip it entirely when
missing. No layout change for documents that do have a title.

- ${center(title.toUpperCase()) ?? \"\"}
+ ${title ? center(title.toUpperCase()) : \"\"}

- # ${title.toUpperCase() ?? \"\"}
+ ${title ? \"# \" + title.toUpperCase() : \"\"}

Test plan

• Added `test/test_export_empty.js`: loads a minimal
  `<story format="mawe" version="6">`,
  flattens it, then runs RTF, HTML, TEX1, TEX2, TXT, MD through
  `flattedFormat` with `assert.doesNotThrow`.
• Wired into the default `npm test` runner via `test/run.mjs`.
• Verified the test catches the regression: stashing only the
  `formatTXT.js` change makes the new test fail with the exact
  `title.toUpperCase()` TypeError described above.
• `npm test` green with the fix applied (`test_load`,
  `test_roundtrip`, and the new `test_export_empty` all pass).