Make tags metadata optional per page

[ruralocity]

After upgrading Jekyll from 1.1.2j to 3.8.3 and jekyll-tagging from 1.0.1 to
1.1.0, having pages with no tags metadata causes builds to fail:

$ jekyll build
Configuration file: /Users/asumner/code/everyday-rails/_config.yml
Source: /Users/asumner/code/everyday-rails
Destination: ./_site

Incremental build: disabled. Enable with --incremental
Generating...
Liquid Exception: undefined method first' for nil:NilClass in /_layouts/post.html jekyll 3.8.3 | Error: undefined method first' for nil:NilClass

Fix by making the tags metadata optional per page. If the "tags" option isn't
found, don't try to make a tags list.