Fix bug in Linter::UnnecessaryStringOutput when tag is empty

This fixes a bug in linter.rb which is exposed in
Linter::UnnecessaryStringOutput where certain helper methods assumed
non-nil input.

Change-Id: Ic11f0ea7a40bffd9689ae58e069ead2827912e75
Reviewed-on: http://gerrit.causes.com/42482
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@brigade.com>
Add Linter::UnnecessaryStringOutput bug fix to changelog

The previous commit fixed a bug but did not mention it in the changelog.
We want people to have a clear understanding of what each release
contains, so I added this change to the changelog.

Change-Id: I56422c508c24c0b8c1bf4d315304217ed1802a50
Reviewed-on: http://gerrit.causes.com/42483
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@brigade.com>
This adds support for excluding frontmatter settings common in many
static site generators like Middleman and Jekyll. It's controlled by a
new global config option 'skip-frontmatter', which defaults to false
so it's opt-in.