[w3c/manifest] fix: purpose member type check + empty string handling (#775)

Closes #771 

This change (choose one):

* [ ] Breaks existing normative behavior (please add label "breaking")
* [X] Adds new normative requirements
* [ ] Adds new normative recommendations or optional items
* [ ] Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
* [ ] Is a "chore" (metadata, formatting, fixing warnings, etc).

Implementation commitment (delete if not making normative changes):

* [ ] Safari (link to issue)
* [x] Chrome - current behavior as per #771 
* [x] [Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=1559412)
* [ ] Edge (public signal)

Commit message:
Clarifies behavior for purpose member type check + empty string handling (to match Chrome):

 * empty string = treat as "any".
 * type check fails = treat as "any".

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/775

-- Commit Summary --

  * fixup 'purpose' member processing
  * fix: purpose member type check + empty string handling

-- File Changes --

    M index.html (39)

-- Patch Links --

https://github.com/w3c/manifest/pull/775.patch
https://github.com/w3c/manifest/pull/775.diff

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