Feed parsing: use author email when there's no author name (#1801)

This is especially useful because when author is given as
`<author>Author Name</author>` ( as in this example
https://cyber.harvard.edu/rss/rss.html#ltauthorgtSubelementOfLtitemgt ),
SimplePie will expose *Author Name* as `email`.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f