Zero front matter should be treated identically to Empty front matter

[vassudanagunta]

Currently a file with the contents:

```

```
page with empty front matter

is not treated identically to a file with the contents:

page with no front matter

Specifically, the logic in Page.update() is executed for the former case, but skipped for the latter. At the moment, the only impact is that the useModTimeAsFallback config setting is ignored for the page with no front matter. This may be minor, but if proposed new functionality for additional default logic for Date and Title are implemented, it will matter a lot more.

[github-actions]

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