Empty slug causes generation of hidden file

[foresto]

Having this line in my markdown source:

Slug:

Causes pelican to generate a file called .html in the output directory. Being a "dot file", it is hidden on unix-like systems, and probably not something that anyone wants. I think it would make more sense to behave as if there was no Slug: line at all.

[justinmayer]

@foresto: I agree. Is this something you would like to help us fix?

[foresto]

Yes, I think I can manage that.

Looking through readers.py and contents.py, I see there are several other metadata that are treated as present even if they are empty. Maybe I should fix them all. Should empty strings be considered valid for any of them?

[foresto]

See also: #1398.

[foresto]

Sorry about all the commit noise. Fixing this without breaking something else turned out to be trickier than I expected, and git's downright hostile UI and documentation have made learning it something of an exercise in trial and error.

In any case, I believe my latest commit does the job. There's a detailed explanation in pull request #1491.

[foresto]

Also, I think it's safe to remove the awaiting user feedback tag. :)