Ignore empty metadata.

[foresto]

I originally set out to stop pelican from creating files named .html when an empty Slug: definition appeared in a Markdown file, but after browsing the reader code, I saw there were more opportunities for empty values to slip in. Since I don't know of any metadata that would make sense with empty values, I made read_file() filter out any such items after it gathers the metadata.

The code filters out most non-true values, including empty strings and None, but it preserves numeric zero values. It runs after the cache phase, so as not to affect caching behavior when whitespace changes are made to a file.

Fixes #1469.
Might also fix #1398?

[foresto]

I noticed a problem in my patch. I'll fix it and re-submit.

[justinmayer]

Is there some reason you closed your pull request instead of rebasing? From project maintainers' perspective, making your follow-on changes and then rebasing+squashing your commits would be preferable.

[foresto]

Only because I just started using git, hadn't yet learned squashing, and didn't want anyone wasting their time reviewing my faulty patch before my revision is ready. I'll squash next time.