Null check font family name in createFontFamilyValue

Font family names can be null by simply providing an empty <string>.
In this case avoid the caching and return an empty CSSFontFamilyValue.

Added computed-style-empty-font-family.html to test the behavior.

Behavior matches Firefox.

BUG=596015

Review URL: https://codereview.chromium.org/1823073003

Cr-Commit-Position: refs/heads/master@{#382745}
3 files changed